page_title: United States vehicle emission standards
section_title: Existing vehicle emissions standard
hierarchical_section_title: United States vehicle emission standards / Existing vehicle emissions standard
caption_attribution_description: EPA Certification test site for 20 degree Fahrenheit fuel economy and emissions testing.
context_page_description: United States vehicle emission standards are set through a combination of legislative mandates enacted by Congress through Clean Air Act amendments of 1970 and beyond, and executive regulations managed nationally by the Environmental Protection Agency, and more recently along with the National Highway Traffic Safety Administration. These standard cover common motor vehicle air pollution, including carbon monoxide, nitrogen oxides, and particulate emissions, and newer versions have incorporated fuel economy standards.
  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 gotten waivers to follow California's standards, which have also become a de facto standard for vehicle manufacturers to follow.
context_section_description: 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.
caption: A representative dynamometer-based emissions test station
exif: {"Image ImageWidth": "6000", "Image ImageLength": "4000", "Image BitsPerSample": "[8, 8, 8]", "Image PhotometricInterpretation": "2", "Image Make": "NIKON CORPORATION", "Image Model": "NIKON D7100", "Image Orientation": "Horizontal (normal)", "Image SamplesPerPixel": "3", "Image XResolution": "300", "Image YResolution": "300", "Image ResolutionUnit": "Pixels/Inch", "Image Software": "Adobe Photoshop CS5.1 Windows", "Image DateTime": "2014:10:17 14:17:05", "Image YCbCrPositioning": "Co-sited", "Image Copyright": "", "Image ExifOffset": "376", "GPS GPSVersionID": "[2, 3, 0, 0]", "Image GPSInfo": "1104", "Thumbnail Compression": "JPEG (old-style)", "Thumbnail XResolution": "72", "Thumbnail YResolution": "72", "Thumbnail ResolutionUnit": "Pixels/Inch", "Thumbnail JPEGInterchangeFormat": "1218", "Thumbnail JPEGInterchangeFormatLength": "0", "EXIF ExposureTime": "1/80", "EXIF FNumber": "63/10", "EXIF ExposureProgram": "Aperture Priority", "EXIF ISOSpeedRatings": "1000", "EXIF SensitivityType": "Recommended Exposure Index", "EXIF ExifVersion": "0230", "EXIF DateTimeOriginal": "2014:10:09 00:47:23", "EXIF DateTimeDigitized": "2014:10:09 00:47:23", "EXIF ComponentsConfiguration": "YCbCr", "EXIF CompressedBitsPerPixel": "4", "EXIF ShutterSpeedValue": "790241/125000", "EXIF ApertureValue": "331919/62500", "EXIF ExposureBiasValue": "0", "EXIF MaxApertureValue": "4", "EXIF SubjectDistance": "89/50", "EXIF MeteringMode": "Pattern", "EXIF LightSource": "Unknown", "EXIF Flash": "Flash fired, compulsory flash mode, return light detected", "EXIF FocalLength": "27", "EXIF SubSecTime": "60", "EXIF SubSecTimeOriginal": "60", "EXIF SubSecTimeDigitized": "60", "EXIF FlashPixVersion": "0100", "EXIF ColorSpace": "sRGB", "EXIF ExifImageWidth": "800", "EXIF ExifImageLength": "534", "Interoperability InteroperabilityIndex": "R98", "Interoperability InteroperabilityVersion": "[48, 49, 48, 48]", "EXIF InteroperabilityOffset": "1072", "EXIF SensingMethod": "One-chip color area", "EXIF FileSource": "Digital Camera", "EXIF SceneType": "Directly Photographed", "EXIF CVAPattern": "[2, 0, 2, 0, 0, 1, 1, 2]", "EXIF CustomRendered": "Normal", "EXIF ExposureMode": "Auto Exposure", "EXIF WhiteBalance": "Auto", "EXIF DigitalZoomRatio": "1", "EXIF FocalLengthIn35mmFilm": "40", "EXIF SceneCaptureType": "Standard", "EXIF GainControl": "High gain up", "EXIF Contrast": "Normal", "EXIF Saturation": "Normal", "EXIF Sharpness": "Normal", "EXIF SubjectDistanceRange": "0"}