EPA-420-F-10-031

 May 2010

National Clean Diesel Campaign

www.epa.gov/cleandiesel

Diesel Oxidation Catalyst

General Information

Technical Bulletin

Technical Overview 

Diesel Oxidation Catalysts, also known as DOCs, are 

exhaust aftertreatment devices that reduce emissions 

from diesel fueled vehicles and equipment. Engine 

manufacturers have used DOCs in different in-use 

applications for many years, and DOCs are widely used as 

a retrofit technology because of their simplicity and limited 

maintenance requirements. DOCs generally consist of a 

precious metal coated flow-through honeycomb structure 

contained in a stainless steel housing. As hot diesel 

exhaust flows through the honeycomb structure, the 

precious metal coating causes a catalytic reaction that 

breaks down pollutants into less harmful components. 

Emissions Reduction 

The United States Environmental Protection Agency 

(EPA) and the California Air Resources Board (CARB) 

evaluate the emission reduction performance of DOCs 

and identify engine operating criteria and conditions that 

must exist for DOCs to achieve those reductions. 

DOCs verified by EPA and CARB are typically effective 

at reducing emissions of particulate matter (PM) by 20 

to 40 percent. EPA’s Verified Technology List also shows 

that DOCs may reduce hydrocarbons by 40 to 75 percent 

and carbon monoxide by 10 to 60 percent. The PM 

removed by DOCs is largely the soluble organic fraction 

that comes from unburned fuel and oil. DOCs generally 

have little impact on elemental carbon and oxides of 

nitrogen (NOx) emissions. DOCs have also been verified 

in combination with crankcase ventilation systems for 

additional emissions reduction. 

EPA is aware of concerns that DOCs may increase the 

nitrogen dioxide (NO2) fraction of total NOx emissions. 

The NO2 produced by a DOC is dependent on the catalyst 

formulation. EPA and CARB have established a limit on 

increases in NO2 emissions from diesel retrofit devices 

and all DOCs on the lists of verified products comply with 

this limit. 

Application 

Verified DOCs are available for nonroad and highway 

heavy-duty diesel engines including those on buses, 

school buses, trucks, mining equipment, construction 

equipment, cargo handling equipment, marine vessels, 

auxiliary power units and stationary generators. 

Each DOC is verified for use with specific engines and/or 

with specific configurations over a range of model years. In 

addition to vehicle and engine specifications, the intended 

application should be evaluated for exhaust temperature, 

fuel sulfur levels and lubrication oil consumption. EPA 

and CARB’s lists of verified diesel retrofit technologies 

define the specific engine operating criteria required 

to successfully apply a particular retrofit technology: 

www.epa.gov/otaq/retrofit/verif-list.htm. 

Fuel 

DOCs perform best with Ultra Low Sulfur Diesel fuel 

(ULSD), and some DOCs are verified for use with Low 

Sulfur Diesel (LSD). ULSD, which contains up to 15 parts 

per million sulfur, is required for highway vehicles and will 

begin to be phased in for the nonroad sector beginning in 

2010.

EPA is aware of concerns that DOCs may release some 

ultrafine particulates. Such concerns are associated with 

high sulfur levels in diesel fuel and the potential for sulfur 

Diesel Oxidation Catalyst (DOC) 
to accumulate in the DOC and then be released as sulfate 

particles. This characteristic may also be associated with 

the precious metal loading and vehicle operation. When 

used with ULSD EPA does not believe DOCs increase 

ultrafine PM. Although nonroad diesel fuel will not be 

required to meet ULSD sulfur levels until 2010, nonroad 

equipment equipped with DOCs should preferably be 

fueled with ULSD. 

Cost 

DOCs generally cost between $600 to $2,000 or more, 

including installation, depending on engine size, installation 

requirements or other unique needs. Because a DOC is 

likely to be heavier than a muffler, it is likely that special 

mounting is necessary. 

Longevity

When properly installed and maintained, DOCs should 

remain effective for the life of the vehicle, generally five 

to ten years or 10,000 or more hours of operation. Engine 

problems with fuel control or oil consumption may quickly 

deteriorate the performance of a DOC. Consequently, 

regular engine maintenance is essential to DOC 

performance.

Warranty coverage is typically part of the commercial 

contract negotiated between the product suppliers and 

their customers. Such warranties typically cover defects in 

materials or workmanship for a specified period defined in 

years, miles and/or operating hours.

As part of their verification program, CARB has established 

detailed warranty periods for CARB-verified retrofit 

technologies as shown in the following table.

Vehicle Category Warranty Period 

GVWR > 33,000 lbs. 

hp > 250 hp and 

miles/year > 100,000 

Vehicle miles < 300k 

Two years; unlimited 

mileage 

GVWR > 33,000 lbs. 

hp > 250 

Five years or 150,000 

miles 

GVWR 19,500 to 

33,000 lbs. 

Five years or 100,000 

miles 

GVWR < 19,000 lbs. Five years or 60,000 

miles 

California Air Resources Board

Warranty Period

Refuse Truck with 

Diesel Oxidation Catalyst (DOC)