# United States vehicle emission standards

United States vehicle emission standards are set through a combination of legislative mandates enacted by Congress through Clean Air Act (CAA) amendments from 1970 onwards, and executive regulations managed nationally by the Environmental Protection Agency (EPA), and more recently along with the National Highway Traffic Safety Administration (NHTSA). These standards cover tailpipe pollution, including carbon monoxide, nitrogen oxides, and particulate emissions, and newer versions have incorporated fuel economy standards. However they lag behind European emission standards, which limit air pollution from brakes and tires.
In nearly all cases, these agencies set standards that are expected to be met on a fleet-wide basis from automobile and other vehicle manufacturers, with states delegated to enforce those standards but not allowed to set stricter requirements. California has generally been the exception, having been granted a waiver and given allowance to set stricter standards as it had established its own via the California Air Resources Board prior to the 1970 CAA amendments. Several other states have since also received waivers to follow California's standards, which have also become a de facto standard for vehicle manufacturers to follow.
Vehicle emission standards have generally been points of debate between the government, vehicle manufacturers, and environmental groups, and has become a point of political debate.

## Legislative and regulation history
### Clean Air Act of 1963 (CAA)

The Clean Air Act of 1963 (CAA) was passed as an extension of the Air Pollution Control Act of 1955, encouraging the federal government via the United States Public Health Service under the then-Department of Health, Education, and Welfare (HEW) to encourage research and development towards reducing pollution and working with states to establish their own emission reduction programs. The CAA was amended in 1965 with the Motor Vehicle Air Pollution Control Act (MVAPCA) which gave the HEW Secretary authority to set federal standards for vehicle emissions as early as 1967. By 1967, this authority empowered the federal government to establish enforceable emission standards, setting a precedent for addressing transportation-related air pollution on a national scale.

### California Air Resources Board (CARB)

In the mid-20th century, California's economy grew rapidly after the Great Depression, but this economic development was accompanied by an increase in air pollution in the state. As a result, smog started to form in the valleys of Southern California, causing respiratory problems for humans and damaging crops.In the 1960s, Dutch chemist Arie Jan Haagen-Smit identified the air pollutants responsible for the smog: carbon monoxide, hydrocarbons, and nitrogen oxides emitted from cars and factories through inefficient fuel combustion. Haagen-Smit also discovered that these air pollutants react with sunlight to form ozone, a major component of smog. As a response to this situation, the California Air Resources Board (CARB) was established in 1967 with Haagen-Smit as its first chairman. CARB set stringent vehicle emission standards to reduce air pollution in the state. California established the California Air Resources Board (CARB) in 1967, with Haagen-Smit as its first chairman, which among other activities set stringent vehicle emission standards by that year.
Other states were also facing similar air pollution issues at the same time, but fearing that setting too strict a standard would drive away automobile manufacturers, they considered implementing standards that were less restrictive compared to California, potentially creating a patchwork of regulations across the United States. The automobile industry lobbied to Congress, and the CAA was modified in 1967 with the National Emissions Standards Act (also known as the Air Quality Act) that expressly prevented states from setting more restrictive emission standards than the federal levels. However, because California has already established its program, it was granted a waiver and allowed to keep its standards. This Act did give states the authority to perform vehicle inspections programs beyond the requirements for new vehicles, though few states took their own action on this.

### Formation of the Environmental Protection Agency and the Clear Air Act Amendment of 1970

Air pollution had become a major national focal point by 1970, leading to a major amendment to the CAA. Near the end of 1970, the United States Environmental Protection Agency (EPA) was formed out of an executive order under President Richard Nixon with ratification by Congress to consolidate all of the environmental-related executive-branch programs to a single entity; the new agency was the primary agency for administering the CAA going forward. Among the provisions related to vehicle emissions:
* The 1970 Amendment required EPA to define the National Ambient Air Quality Standards (NAAQS), to be set and updated by the EPA. The NAAQS, at passage of the 1970 amendment, included the pollutants carbon monoxide, nitrogen dioxide, sulfur dioxide, particulate matter, hydrocarbons and photochemical oxidants (such as ozone). Other pollutants, such as lead, were added later based on EPA review of current conditions. Along with the other activities set by the EPA, the states were to create State Implementation Plans (SIP)s to bring their air quality within the NAAQS by 1975.
* The Amendment also required the EPA to define emission standards for new vehicles to help with the NAAQS emission reduction goals, including standards for fuel and testing of new vehicles to make sure these standards are met.
Additionally, the 1970 CAA Amendment continued California's waiver program through which California can seek exemptions from the EPA's emissions requirements as long as theirs are at least as strict as the EPA's vehicle standards.

### Clear Air Act Amendment of 1977

The EPA's assessment of the state of the country meeting the target NAAQS goals by 1975 was poor, having identified numerous nonattainment areas in the country. With the 1977 Amendment to the CAA, a new deadline of December 31, 1982, for meeting the NAAQS was fixed with no allowance for extending the deadline unless specific control measures were established. Among other key provisions was the establishment of required vehicle inspection and maintenance programs (I/M) in nonattainment states and optional in other areas. This required that states establish emission testing facilities for in-use vehicles to make sure they meet emissions requirements, maintained and repaired as necessary to correct any problems before their license was renewed. The EPA was tasked to establish the basic protocols for these facilities. Other states that had met the NAAQS attainment goals could optionally establish I/M programs for existing but were required to follow the EPA's specifications.

## New vehicle emission standards

Due to its preexisting standards and particularly severe motor vehicle air pollution problems in the Los Angeles metropolitan area, the U.S. state of California has special dispensation from the federal government to promulgate its own automobile emissions standards. Other states may choose to follow either the national standard or the stricter California standards. The states that have adopted the California standards are: Colorado, Connecticut, Delaware, Maine, Maryland, Massachusetts, New Jersey, New Mexico (2011 model year and later), New York, Nevada, Oregon, Pennsylvania, Rhode Island, Vermont, and Washington (2009 model year and later), as well as the District of Columbia. Such states are frequently referred to as "CARB states" in automotive discussions because the regulations are defined by the California Air Resources Board.
The EPA adopted the Californian fuel economy and greenhouse gas standard as a national standard by the 2016 model year and collaborated with Californian regulators on stricter national emissions standards for model years 2017–2025.

### Criteria pollutants
#### Light-duty vehicles

Light-duty vehicles are certified for compliance with emission standards by measuring their tailpipe emissions during rigorously-defined driving cycles that simulate a typical driving pattern. The FTP-75 city driving test (averaging about 21 miles per hour (34 km/h)) and the HWFET highway driving test (averaging about 48 miles per hour (77 km/h)) are used for measuring both emissions and fuel economy.
Two sets, or tiers, of emission standards for light-duty vehicles in the United States were defined as a result of the Clean Air Act Amendments of 1990. The Tier I standard was adopted in 1991 and was phased in from 1994 to 1997. Tier II standards were phased in from 2004 to 2009.
Within the Tier II ranking, there is a subranking ranging from BIN 1–10, with 1 being the cleanest (Zero Emission vehicle) and 10 being the dirtiest. The former Tier 1 standards that were effective from 1994 until 2003 were different between automobiles and light trucks (SUVs, pickup trucks, and minivans), but Tier II standards are the same for both types.
These standards specifically restrict emissions of carbon monoxide (CO), oxides of nitrogen (NO$_x$), particulate matter (PM), formaldehyde (HCHO), and non-methane organic gases (NMOG) or non-methane hydrocarbons (NMHC). The limits are defined in grams per mile (g/mi).

##### Phase 1: 1994–1999

These standards were phased in from 1994 to 1997 and were phased out in favor of the national Tier 2 standard from 2004 to 2009.
Tier I standards cover vehicles with a gross vehicular weight rating (GVWR) below 8,500 pounds (3,856 kg) and are divided into five categories: one for passenger cars, and four for light-duty trucks (which include SUVs and minivans) divided up based on the vehicle weight and cargo capacity.
California's Low-emission vehicle (LEV) program defines six automotive emission standards which are stricter than the United States' national Tier regulations. Each standard has several targets depending on vehicle weight and cargo capacity; the regulations cover vehicles with test weights up to 14,000 pounds (6,400 kg). Listed in order of increasing stringency, the standards are:
* TLEV – Transitional low-emission vehicle
* LEV – Low-emission vehicle
* ULEV – Ultra-low-emission vehicle
* SULEV – Super-ultra low-emission vehicle
* ZEV – Zero-emission vehicle
The last category is largely restricted to electric vehicles and hydrogen cars, although such vehicles are usually not entirely non-polluting. In those cases, the other emissions are transferred to another site, such as a power plant or hydrogen reforming center, unless such sites run on renewable energy.

##### Transitional NLEV: 1999–2003

A set of transitional and initially voluntary "national low emission vehicle" (NLEV) standards were in effect starting in 1999 for northeastern states and 2001 in the rest of the country until Tier II, adopted in 1999, began to be phased in from 2004 onwards. The National Low Emission Vehicle program covered vehicles below 6,000 pounds (2,700 kg) GVWR and adapted the national standards to accommodate California's stricter regulations.

##### Phase 2: 2004–2009

Instead of basing emissions on vehicle weight, Tier II standards are divided into several numbered "bins". Eleven bins were initially defined, with bin 1 being the cleanest (zero-emission vehicle) and 11 the dirtiest. However, bins 9, 10, and 11 are temporary. Only the first ten bins were used for light-duty vehicles below 8,500 pounds (3,900 kg) GVWR, but medium-duty passenger vehicles up to 10,000 pounds (4,500 kg) GVWR and to all 11 bins. Manufacturers can make vehicles which fit into any of the available bins, but still must meet average targets for their entire fleets.
The two least-restrictive bins for passenger cars, 9 and 10, were phased out at the end of 2006. However, bins 9 and 10 were available for classifying a restricted number of light-duty trucks until the end of 2008, when they were removed along with bin 11 for medium-duty vehicles. As of 2009, light-duty trucks must meet the same emissions standards as passenger cars.
Tier II regulations also defined restrictions for the amount of sulfur allowed in gasoline and diesel fuel, since sulfur can interfere with the operation of advanced exhaust treatment systems such as selective catalytic converters and diesel particulate filters. Sulfur content in gasoline was limited to an average of 120 parts-per-million (maximum 300 ppm) in 2004, and this was reduced to an average 30 ppm (maximum 80 ppm) for 2006. Ultra-low sulfur diesel began to be restricted to a maximum 15 ppm in 2006 and refiners are to be 100% compliant with that level by 2010.
A second round of California standards, known as Low Emission Vehicle II, is timed to coordinate with the Tier 2 rollout.
The PZEV and AT-PZEV ratings are for vehicles which achieve a SULEV II rating and also have systems to eliminate evaporative emissions from the fuel system and which have 150,000-mile/15-year warranties on emission-control components. Several ordinary gasoline vehicles from the 2001 and later model years qualify as PZEVs. If a PZEV has technology that can also be used in ZEVs like an electric motor or high-pressure gaseous fuel tanks for compressed natural gas (CNG) or liquified petroleum gas (LPG), it qualifies as an AT-PZEV.
Diesel particulate filters became a requirement in 2014; gasoline vehicles were exempt.

##### Tier III

The phase-in of new tailpipe and evaporative emission standards begin to phase-in beginning with the 2017 model year along with new fuel standards.

##### Tier IV

New standards were set in March 2024 to be phased in beginning with the 2027 model year.

#### Heavy-duty vehicles

Heavy-duty vehicles must comply with more stringent exhaust emission standards and requires ultra-low sulfur diesel (ULSD) fuel (15 ppm maximum) beginning in 2007 model year.

### Greenhouse gases

Federal emissions regulations cover the primary component of vehicle exhaust, carbon dioxide (CO$_2$). Since CO$_2$ emissions are proportional to the amount of fuel used, the national Corporate Average Fuel Economy (CAFE) regulations were historically the primary way in which automotive CO$_2$ emissions were regulated in the U.S. The EPA faced a lawsuit seeking to compel it to regulate greenhouse gases as a pollutant, Massachusetts v. Environmental Protection Agency.
As of 2007, the California Air Resources Board passed strict greenhouse gas emission standards which are being challenged in the courts.
On September 12, 2007, a judge in Vermont ruled in favor of allowing states to conditionally regulate greenhouse gas (GHG) emissions from new cars and trucks, defeating an attempt by automakers to block state emissions standards. A group of automakers including General Motors, DaimlerChrysler, and the Alliance of Automobile Manufacturers had sued the state of Vermont to block rules calling for a 30 percent reduction in GHG emissions by 2016. Members of the auto industry argued that complying with these regulations would require major technological advances and raise the prices of vehicles as much as $6,000 per automobile. U.S. District Judge William K. Sessions III dismissed these claims in his ruling. "The court remains unconvinced automakers cannot meet the challenge of Vermont and California's (greenhouse gas) regulations," he wrote.
Environmentalists pressed the Administration to grant California a waiver from the EPA for its emissions standards to take effect. Doing so would allow Vermont and other states to adopt these same standards under the Clean Air Act. Without such a waiver, Judge Sessions wrote, the Vermont rules will be invalid.

#### Light-duty vehicles
##### 2010–2016

In 2009, President Obama announced a new national fuel economy and emissions policy that incorporated California's contested plan to curb greenhouse gas emissions on its own, apart from federal government regulations.
The standards are formatted such that each vehicle has an emissions target as a function of the product of its wheelbase and average track width with separate functions for passenger cars and light trucks with progressively smaller targets by model year. Thus each manufacturer has a unique standard for each model year based on the characteristics of vehicles it actually produces. The new standards established a credit trading system whereby manufacturers that overperform their annual target may sell credits to other manufacturers which they then may use to meet a credit shortfall it has from failing to meet its standards through emissions improvements.
The combined fleet fuel economy for new cars and trucks with a GVWR of 10,000 pounds (4,500 kg) or less was projected to average 35.5 miles per gallon (mpg) for the 2016 model year based on the newly-established targets and projected fleet mix. The average for its cars will have to be 42 mpg, and for its trucks will be 26 mpg by 2016, in coordination with new CAFE standards. If the average fuel economy of a manufacturer's annual fleet of vehicle production falls below its defined standard, the manufacturer must pay a penalty, then US$5.50 per 0.1 mpg under the standard, multiplied by the manufacturer's total production for the U.S. domestic market. This is in addition to any gas guzzler tax, if applicable.
Should CAFE targets have been extended through to 2026 under the Obama administration, it would have sought a 54 mpg industry-wide average fuel efficiency for cars and light trucks manufactured in 2026 or later, with automobile manufacturers instructed to increase the fuel economy across all of their vehicles by 5% each year.

##### Trump-era rollback and Biden-era reversal (2017–2021)

After Donald Trump was inaugurated as president in 2017, he instructed the NHTSA and EPA to rollback Obama's CAFE standards, increasing the 2026 target to a then-projected 202 g CO$_2$/mi and requiring only an annual 1.5% fleet efficiency improvement. The new rule was issued in March 2020. The Trump administration argued the rollback was required due to the increasing costs of cars on consumers that higher efficiencies would only make more expensive. The move was criticized by several environmentalists, Consumer Reports, as well as the state of California, as the ruling coincided with Trump's efforts to remove the waiver for California emissions exemptions.
Following Joe Biden becoming president in 2021, he signed Executive Order 14057, "Catalyzing Clean Energy Industries and Jobs Through Federal Sustainability", which in addition to committing the federal government to implement clean transport options such as EVs, also committed to improving the fuel efficiency standards and reversing the Trump administration's actions. The EPA issued a new rule in December 2021, to become enforceable by February 2022, that effectively restored the Obama-era standards, through decreasing the fleet-wide emissions target to a projected 161 g CO$_2$/mi by the 2026 model year.

### Consumer ratings
#### Air pollution score

EPA's air pollution score represents the amount of health-damaging and smog-forming airborne pollutants the vehicle emits. Scoring ranges from 0 (worst) to 10 (best). The pollutants considered are nitrogen oxides (NOx), particulate matter (PM), carbon monoxide (CO), formaldehyde (HCHO), and various hydrocarbon measures – non-methane organic gases (NMOG), and non-methane hydrocarbons (NMHC), and total hydrocarbons (THC). This score does not include emissions of greenhouse gases (but see Greenhouse gas score, below).

#### Greenhouse gas score

EPA's greenhouse gas score reflects the amount of greenhouse gases a vehicle will produce over its lifetime, based on typical consumer usage. The scoring is from 0 to 10, where 10 represents the lowest amount of greenhouse gases.
The Greenhouse gas score is determined from the vehicle's estimated fuel economy and its fuel type. The lower the fuel economy, the more greenhouse gas is emitted as a by-product of combustion. The amount of carbon dioxide emitted per liter or gallon burned varies by fuel type, since each type of fuel contains a different amount of carbon per gallon or liter.
The ratings reflect carbon dioxide (CO$_2$), nitrous oxide (N$_2$O) and methane (CH$_4$) emissions, weighted to reflect each gas's relative contribution to the greenhouse effect.

### California emission standards

Under Section 209 of the Clean Air Act (CAA), California is given the ability to apply for special waivers to apply its own emission standards for new motor vehicles that are at least as stringent as the federal standards. California applies for this waiver through the EPA, which publishes the proposed standards for public review in the Federal Register. Based on its own review and public comments, the EPA then grants the waiver unless it has determined that California's requested standards were "arbitrary and capricious" in their findings, that the standards are not needed to "meet compelling and extraordinary conditions", or otherwise are inconsistent with other aspects of the CAA.
Since the CAA's passage in 1967, California has applied and received more than fifty waivers, which include emission standards across various vehicle classes. Among these include two special sets of waivers:
* California initiated its zero-emission vehicle (ZEV) mandate in 1990. ZEV are defined as vehicles that have no exhaust or evaporative emissions of any regulated pollutant. Vehicle manufacturers were a percentage of their fleet meeting these ZEV standards over a long-term schedule (2% by model year 1998 at its start), but the mandate schedule has shifted based on the unplanned rate of technology advancement and costs, and as of 2020, its current target is to reach 8% ZEV by 2025 determined by fleet credits that account for vehicle range as well as contributions from any low-emission vehicles or plug-in hybrids. The EPA granted the initial request in 1990 and several updates.
* California had requested to regulate greenhouse gas emissions (GHG) at stricter levels than federal levels first in 2005 as part of its low-emission vehicle program. The EPA initially refused this waiver based on a decision from the United States Court of Appeals for the District of Columbia Circuit that determined the EPA did not have the authority to regulate GHG under the CAA; this ruling was challenged in the Supreme Court case Massachusetts v. Environmental Protection Agency (549 U.S. 497 (2007)) which ruled that the EPA did have this authority. In later actions, the EPA granted California its GHG waiver by 2009.
On September 21, 2021, CARB proposed a plan to upgrade all engines in cars, trucks, and boats to EPA "Tier 3" levels by the end of 2024, and the "Tier 4" level, which is more restrictive, by 2035. Tier 4 engine technology does not yet exist. The Sportfishing Association of California and Golden Gate Fishermen’s Association argued against this proposal on the basis on the upgrades to their boats would be economically disastrous and would lead to charging their customers higher ticket cost, and affect their ability to stay in business. The two sides reached a compromise to upgrade to Tier 3 and delay the Tier 4 update for when the technology is available and cost effective.
In November 2022, CARB finalized a rule for MY 2026 through 2035 called Advanced Clean Cars II (ACCII). ZEV sales requirements, starting from 35% to 100%, are for manufacturers with annual sales between 4,501 and 60,000 vehicles. 100% ZEV mandate by 2035, includes up to 20% of PHEVs by referring them as TZEV (Transitional Zero Emission Vehicle) and rest of the proportion with BEVs and FCEVs.

#### State adoption of California Standards

Section 177 of the CAA grants the ability for states to adopt California emission standards instead of federal ones. As of December 2021, the following states have adopted the California standards, including their standards for ZEV and GHG:
| State | Adopted (by model year) | Adopted (by model year) | Adopted (by model year) |
| State | Criteria Pollutant Regulation | GHG Regulation | ZEV Program |
| California | 1992 | 2009 | 1990 |
| New York | 1993 | 2009 | 1993 |
| Massachusetts | 1995 | 2009 | 1995 |
| Vermont | 2000 | 2009 | 2000 |
| Maine | 2001 | 2009 | 2001 |
| Pennsylvania | 2001 | 2009 | — |
| Connecticut | 2008 | 2009 | 2008 |
| Rhode Island | 2008 | 2009 | 2008 |
| Washington | 2009 | 2009 | — |
| Oregon | 2009 | 2009 | 2009 |
| New Jersey | 2009 | 2009 | 2009 |
| Maryland | 2011 | 2011 | 2011 |
| Delaware | 2014 | 2014 | — |
| Colorado | 2022 | 2022 | 2023 |
| Nevada | 2025 | 2025 | 2025 |
| Virginia | 2025 | 2025 | 2025 |
* ↑ Washington passed legislation to meet California's ZEV standards by model year 2022 in March 2020

#### Revocation of waivers under the Trump administration

Former President Donald Trump stated his concern about California's stricter emission standards and their impact on the costs of manufacturing on the automobile industry, though some political analysts asserted this also tied in with Trump's conservative ideology conflicting with California's more liberal stance. Along with the Obama-era mileage goals, Trump had expressed his intent to revoke California's waivers early on in his presidency.
Shortly after Ford, Volkswagen, Honda, and BMW announced their intentions to commit to the Obama-era mileage goals and California's emission standards across their fleets in July 2019, Trump announced his intention to rollback California's waivers. As part of Trump's "Safer, Affordable, Fuel-Efficient" (SAFE) program, the EPA and NHTSA proposed a new "One National Program Rule" that asserted that only the federal government may set emissions standards on September 19, 2019, as to have one consistent set of fuel emission and mileage standards across the country. This rule would include revoking the last set of California waivers that the EPA had granted California in 2013 for its GHG and ZEV programs. California retained its ability to set emission standards that address ozone-formation under the rule.
Subsequent to this rule, California led a collation of 23 states to sue the NHTSA in California v. Chao (Case 1:19-cv-02826) in the D.C. District Court in September 2019, asserting the agency, in setting the rule, violated the intent of the CAA. The same group of states also filed suit against the EPA once the EPA issued the revoking of the 2013 waiver in November 2019, in California v. Wheeler (Case 19-1239) in the United States Court of Appeals for the District of Columbia Circuit to challenge the EPA's revoking. Further, both Minnesota and New Mexico, plaintiffs in both cases, stated they would take steps to adapt California's standards in their states as a result.
Following the election of Joe Biden as president, the EPA and NHTSA moved to reverse the 2019 rule in April 2021, thus returning to the previous status quo for California. Following the EPA granting California its latest request for exemption, seventeen states sued the EPA in May 2022, arguing that because of the impact of California emission standards on vehicle manufacturing, the EPA's actions violate the equal sovereignty granted to the states by the Constitution, since it gives California more power than other states in setting emissions regulations.

## Non-road engines

Non-road engines, including equipment and vehicles that are not operated on the public roadways, are used in an extremely wide range of applications, each involving great differences in operating characteristics and engine technology. Emissions from all non-road engines are regulated by categories.
In the United States, the emission standards for non-road diesel engines are published in the US Code of Federal Regulations, Title 40, Part 89 (40 CFR Part 89). Tier 1–3 Standards were adopted in 1994 and was phased in between 1996 and 2000 for engines over 37 kW (50 hp). In 1998 the regulation included engines under 37 kW and introduced more stringent Tier 2 and Tier 3 standards which was scheduled to be phased in between 2000 and 2008. In 2004, US EPA introduced the more stringent Tier 4 standards which was scheduled to be phased in between 2008 and 2015. The testing cycles used for certification follow the ISO 8178 standards.

## Small engines

Pollution from small engines, such as those used in gas-powered groundskeeping equipment reduces air quality. Emissions from small offroad engines are regulated by the EPA. Specific pollutants subject to limits include hydrocarbons, carbon monoxide, and nitrogen oxides.

## Existing vehicle emissions standard

Testing of existing vehicle emissions, also known as vehicle inspection and maintenance programs (I/M) were introduced as part of the 1977 Amendments to the CAA. The 1970 Amendments introduced target NAAQS goals for air quality which were not met in many parts of the country. With the 1977 Amendments, the CAA required I/M programs in non-attainment states as part of their pollution prevent plans.
For model years prior to 1996, emissions tests were performed using a chassis dynamometer-based test; The vehicle is driven so that the wheels of its main driven axle (front or rear) sit atop the dynamometer rotors, when then are unlocked to rotate freely. A special collections line is attached to the tailpipe, and simulated airflow is pushed across the engine to simulate vehicle movement. The test operator then presses the accelerator of the car through a fixed test schedule: the acceleration from the engine translates to force and torque that are measured through the dynamometer, simultaneously mapped against analysis of the emissions from the tailpipe. After completion of the schedule, the computerized system calculates the emissions from the car and determines if it meets the appropriate specification for its model year.
Since model year 1994, all LDV and LDT manufactured for use in the United States are required to use the standard on-board diagnostic OBD-II system. This is a computerized system that continually monitors the performance of the engine and its emission control system. Instead of the dynamometer test, the operator hooks the OBD-II to a standard computer system which downloads the information from the computer. It will warn the operator if the OBD-II determines significant deviations from expected emissions control standards, indicating repairs may be needed.
State Emissions :
| Name | Light duty gasoline | Emissions testing | Light-duty diesel | Emissions testing | Heavy-duty diesel | Emissions testing | Special notes |
| Alabama | | No emissions testing required | | No emissions testing required | | | Alabama maintains "volunteer emission testing" |
| Alaska | ≥6 model years old (Anchorage) | Biennially | | Exempt | | Exempt |
| Arizona | Model Years ≥1967 but more than 5 model years old (Pima County/ Maricopa County) | Biennially | 1967–2007 (Pima County/ Maricopa County) | Pima County: 30 percent opacity Maricopa County: 20 percent opacity | Maricopa County | J1667 test protocol Model year <1991: 55 percent opacity Model year ≥1991: 40 percent opacity | Only state which still conduct emission testing dating back to the 1967 model year unlike other states using EPA classification (a few still conduct test for 1968–present (1968+ testing is for jurisdictions using defined EPA standards for vehicle classification since the '68 model year and beyond automobiles must have an exhaust emission system – PCV system, air injection pump, controlled spark module) e.g. the absence of a rolling chassis exemption – in some states when an automobile reaches an age limit (as in 25 model years old) the vehicle is exempted or pertaining to motor vehicle laws e.g. classic automobile registration and displaying vintage license plates |
| Arkansas | | | | | | | |
| California | vehicles 1976 and newer model-year gas-powered, hybrid, and flex-fuel vehicles; exemptions: new car = ≤6 model-years old are exempt; change of ownership = ≤4 model-years old are exempt | required biennially for registration renewal; 2000 model-year and newer receive only visual and OBDII inspections | vehicles 1998 and newer model-year diesel-powered vehicles with a GVWR ≤14,000 pounds; exemptions: none applicable to diesel-powered vehicles | required biennially for registration renewal; 1998 model-year and newer receive only visual and OBDII inspections | California-based fleets of 2 or more heavy-duty vehicles with a GVWR >14,000 pounds | Periodic Smoke Inspection Program (PSIP) an annual smoke and tamper self-inspection; exemptions: new vehicle = ≤4 model-years old are exempt | Prior to 2004 emission testing included 1966 and newer model year automobiles – 1984 calendar year phased in the use of an acceleration simulated mode test criteria – a 1998 state law made pre-1973 automobiles exempt from emission testing where it temporarily included a 30-year rolling chassis exemption which was repealed in 2006 where 1976 and newer are subjected to state mandated emission testing – California visual inspection strictest in the USA (also similar to Japanese 'Shaken' vehicle laws which is their equivalent of vehicle inspection) which serves as a model for other states implementing I/M (inspection/maintenance) programs |
| Colorado | Vehicles older than 4 years required, this will change to 7 years in 2015. Specially registered cars made before 1975 and special vehicles may be exempted. | Biennially when required | Exempt for first seven model years of gasoline vehicles. | When outside exemption, vehicles up to 32 years old – biennially, else annually. OBD testing done when newer than 10 years | Exempt for first four model years of diesel vehicles. | When outside exemption, diesel trucks less than 10 years – Biennially else annually. | Only on high population corridor encompassing Castle Rock to Wellington, including Boulder and Greeley. Required for vehicles used in this area. Dynamometer testing used. |
| Connecticut | more than 4 model years old but less than 25 model years old | Biennially (OBDII) | more than 4 model years old but less than 25 model years old | Biennially | | Exempt | |
| Delaware | Over 5 model years old | Biennially (OBDII) | | | | | |
| Florida | | | | | | | |
| Georgia | | Annually (Atlanta metro area only) | | | | | |
| Hawaii | | | | | | | |
| Idaho | | | | | | | |
| Illinois | | Biennially, after the vehicle is four years old | | | | | Required only in the Chicago metropolitan area and eastern suburbs of St. Louis, Missouri |
| Indiana | | | | | | | |
| Iowa | | | | | | | |
| Kansas | | | | | | | |
| Kentucky | | | | | | | |
| Louisiana | | | | | | | |
| Maine | | | | | | | |
| Maryland | | Biennially | | | | | Hybrid vehicles that achieve an EPA gas mileage rating of at least 50 miles per gallon are exempt from testing for 3 years after their model year, after which they are then tested at the standard two-year intervals. |
| Massachusetts | Every year | OBDII | Every year | OBDII | 8,501 to 14,000 lbs (Medium duty) 14,000 lbs+ (Heavy duty) | Medium Duty: OBDII Heavy duty: J1667 Protocol | |
| Michigan | | | | | | | |
| Minnesota | | | | | | | |
| Mississippi | | | | | | | |
| Missouri | | | | | | | |
| Montana | | | | | | | |
| Nebraska | | | | | | | |
| Nevada | 1968 model year or newer. New vehicles on their third registration; first and second registrations are exempted. Hybrids are exempt for five model years. Motorcycles or mopeds, alternative fuel vehicles, vehicles registered as a Classic Rod, Classic Vehicle or Old Timer and driven 5,000 miles or less per year, vehicles registered as a Replica Vehicle, and fleet vehicles participating in the Continuous Monitoring program are also exempt. | Annually. OBDII testing for 1996 model year or newer; Two-Speed Idle Test for older vehicles. | 1968 model year or newer. New vehicles on their third registration; first and second registrations are exempted. GVWR ≥14,000 exempt, as are full time four wheel drive light duty diesels with a wheelbase ≥182 inches. | Annually. Light duty diesel emission inspections are conducted by using an opacity meter and dynamometer. A visual inspection for tampering of emission control devices is also part of the inspection procedure. | | | Emissions testing is required in: The Las Vegas Valley and within a five-mile buffer zone around it. Reno, Sparks, Washoe Valley and most of the area west of Washoe Valley. Testing is also required in the areas north and east of Reno between Interstate 80 and the 40th degree of north latitude (about midway through Pyramid Lake). Vehicles based in remote areas of Clark and Washoe counties and all other Nevada counties are exempt. |
| New Hampshire | | | | | | | |
| New Jersey | Model years ≥ 1995 Must be inspected biennially. Model years ≤ 1995 do not require inspection. | | | | | | |
| New Mexico | Model Years ≥1978 but more than 4 model years old (Bernalillo County) | Biennially and at change of ownership | Model Years ≥1978 but more than 4 model years old (Bernalillo County) | Biennially and at change of ownership | | Exempt | |
| New York | | | | | | | |
| North Carolina | | | | | | | |
| North Dakota | | | | | | | |
| Ohio | More than 4 model years old but less than 25 model years old in the Greater Cleveland area | Biennially (OBDII) | More than 4 model years old but less than 25 model years old in the Greater Cleveland area | Biennially (OBDII) | | | Testing was also required in the Cincinnati and Dayton Metropolitan areas from 1995 to 2005 |
| Oklahoma | | | | | | | |
| Oregon | Vehicles must be tested every other year for registration renewal in the Portland and Medford areas. Model Years ≥1975 must be tested in Portland area, and those 20 model year and newer in Medford area. Also all vehicles new to Oregon. | Biennially | Same intervals and exemptions as gasoline cars, 8,500 lbs or less | Biennially | ≥8,501 lbs | Exempt | |
| Pennsylvania | | | | | | | |
| Rhode Island | more than 2 models years old (inclusive) or 24,000 miles (inclusive) | Biennially (OBDII) | | Exempt | | Exempt | Effective July 2012, any state contracted job requiring heavy duty vehicles must be operated with pollution control devices |
| South Carolina | | | | | | | |
| South Dakota | | | | | | | |
| Tennessee | Any gasoline vehicle with a model year of 1975 and newer and a GVWR of 10,500 lbs or less. Motorcycles are exempt. | Annual testing. OBDII + catalytic converter tamper check + fuel cap leak test for 1996 or later vehicles. Tailpipe/tampering testing for 1995 and older vehicles. | Any diesel vehicle with a model year of 1975 and newer and a GVWR of 10,500 lbs or less. | Annual testing. OBDII + catalytic converter tamper check + fuel cap leak test for 2002 or later vehicles. Curb Idle-opacity/tampering testing for 2001 and older vehicles. | | Exempt | Only applies to Hamilton (Chattanooga), Davidson, Rutherford, Sumner, Williamson, and Wilson (all part of Greater Nashville area) Counties. Required for registration renewal. Davidson county runs their own emissions programs/facilities with the assistance of the state. Shelby County (Memphis) formerly required testing, but ended their program in July 2013 due to funding issues and no state support. |
| Texas | more than 2 model years old to 24 model years old; motorcycles, diesel-powered, and automobiles 25 model years or older exempt from emission testing – includes motor vehicles registered as an Antique or Classic under Texas law (either displaying state-mandated vanity license plate or vintage-era license plates (this also includes past general issue plates from 1975–present with validation stickers used prior to the 1995–present windshield sticker; license plate law in Texas (effective since May 1988) requires new general issue plates after it reaches 8 years of age – license plate law was amended 11.1.16 where TxDMV no longer mandates plate replacement every 8 years with the exception that the license plate is considered legible e.g. loss of reflectivity with the paint base and/or if a license plate is unreadable if a police officer does a probable cause traffic stop) | annually (both OBDII for 1996–present and ASM to 1995 for motor vehicles registered in the DFW Metroplex and Houston Metro area except for 4-wheel drive/all wheel drive powertrains (4WD/AWD including vehicles over 8500 GVW are subjected to the two-speed idle (TSI) testing; TSI testing to 1995 only in El Paso, Travis, and Williamson Counties) | | | | | Emission testing mandated for motor vehicles registered in the Houston Metropolitan area (Brazoria, Fort Bend, Galveston, Harris, Montgomery), DFW Metroplex (Collin, Dallas, Denton, Ellis, Johnson, Kaufman, Parker, Rockwall, Tarrant), Austin Metropolitan area (Travis, Williamson) and El Paso counties; since March 1, 2015, inspection stickers no longer issued where an automobile must pass the safety or emission test prior to renewing automobile registration |
| Utah | | | | | | | |
| Virginia | more than 2 model years old to 24 model years old | Biennially. First-time registrations exempt if vehicle has received an emissions certificate from certain states within previous 12 months. | newer than 1996 model year | Biennially | GVWR > 8500 lbs. exempt | | Emission testing mandated for motor vehicles registered in the counties of Arlington, Fairfax, Loudoun, Prince William, or Stafford, and the cities of Alexandria, Fairfax, Falls Church, Manassas or Manassas Park. Certain hybrids exempt. |
| Washington | | No emissions testing required | | No emissions testing required | | |
| West Virginia | | No emissions testing required | | No emissions testing required | | | |
| Wisconsin | Testing required for vehicles model years 1996 and later in Kenosha, Milwaukee, Ozaukee, Racine, Sheboygan, Washington and Waukesha counties (model years 1996 to 2006 up to 8,500 pounds; model years 2007 or newer up to 14,000 pounds) | Every other year before registration renewal; after transfer of ownership; after registration in Wisconsin | Testing required for vehicles model years 1996 and later in Kenosha, Milwaukee, Ozaukee, Racine, Sheboygan, Washington and Waukesha counties: Model years 2007 and newer which have a gross vehicle rating up to 14,000 pounds | Every other year before registration renewal; after transfer of ownership; after registration in Wisconsin | Greater than 14,000 lbs. | Exempt | |
| Wyoming | | | | | | | |