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Please note: This summary is provided to help you understand the regulations. Consult the references provided for links to the full text of the regulations.

On-Board Diagnostics

In 2000, the U.S. Environmental Protection Agency (EPA) introduced a new program, the Heavy-Duty Engine and Vehicle Standards and Highway Diesel Fuel Sulfur Control Requirements. Under this program, EPA published:

  • Diesel fuel standards that reduced the level of sulfur in highway diesel fuel by 97%
  • Engine emission standards for new vehicles, based on the use of high-efficiency catalytic exhaust emission control devices or comparably effective advanced technologies.

The diesel fuel standards have been in effect since 2006. The engine emission standards began to be phased in starting with model year 2007.

Of course, the benefits of emissions standards for new vehicles will only be realized if the emission control systems on the vehicles are properly maintained. Accordingly, EPA also published a rule that requires the emissions control systems of large highway diesel and gasoline trucks to be monitored for malfunctions via on-board diagnostic systems (OBD), similar to those systems required on passenger cars since the mid-1990s. This TERC topic page explains the OBD rule.

Who is covered by the regulations?

The OBD regulation affects manufacturers or importers of new heavy-duty vehicles or engines intended for highway use; the rule also applies to anyone who converts heavy-duty vehicles or engines used in highway vehicles to run on alternative fuels.

What is the purpose of the regulations?

As part of the Heavy-Duty Engine and Vehicle Standards and Highway Diesel Fuel Sulfur Control Requirements program, new emission standards for heavy-duty engines and vehicles took effect in model years 2007 through 2010. These standards are based on the use of high-efficiency catalytic exhaust emission control devices or comparably effective advanced technologies. Because these devices are damaged by sulfur, the program also reduces the level of sulfur in highway diesel fuel by 97 percent. The emissions reductions associated with this program are estimated to result in over $70 billion in public health and welfare benefits through reduced hospitalizations and lost work days. The OBD requirements are intended to help to ensure that these benefits are realized by requiring truck manufacturers to install systems that will alert operators to pollution control system malfunctions.


The On-Board Diagnostics rule (Federal Register Vol. 74, No. 35 February 24, 2009) requires manufacturers to install OBD systems that:

  • Monitor the functioning of emission control components
  • Alert the vehicle operator to any detected need for emission-related repair

In addition, when a malfunction occurs, diagnostic information must be stored in the engine's computer to assist in diagnosis and repair of the malfunction. Also, manufacturers are required to make available to the service and repair industry information necessary to perform repair and maintenance service on OBD systems and other emission related engine components. The regulation specifically requires:

  • For 2010 and later model year heavy-duty diesel and gasoline engines used in highway applications over 14,000 pounds, all major emissions control systems (e.g., the diesel particulate filters and oxides of nitrogen (NOx) reducing catalysts) be monitored and malfunctions be detected prior to emissions exceeding a set of emissions thresholds.  The rule also requires that all emission-related electronic sensors and actuators be monitored for proper operation.
  • For highway applications over 14,000 pounds, the rule requires that one engine family per manufacturer be certified to the OBD requirements in the 2010 through 2012 model years. Beginning in 2013, all highway engines for all manufacturers would have to be certified to the OBD requirements. This phase-in is designed to spread over a number of years the development effort required of industry and to provide industry with a learning period prior to implementing the complex OBD requirements on 100 percent of their highway product line.
  • For applications over 14,000 pounds, the service information availability requirements apply for those engines certified to the OBD requirements.
  • For 2010 and later model year highway heavy-duty diesel applications under 14,000 pounds, EPA published a new emissions threshold for monitoring of the diesel particulate filter. The emission threshold is consistent, both in stringency and in timing, with the particulate matter (PM) thresholds for over 14,000 pound applications.
  • For 2007 and later model year diesel highway heavy-duty applications under 14,000 pounds, EPA changed the emission thresholds for NOx emissions. The existing thresholds (typically 1.5 times the applicable NOx standard) were established when the engine's NOx standard (i.e., the 2004 NOx standard) was much higher than today's very low level (i.e., the 2010 NOx standard).

Best Practices

OBD systems help keep diesel emissions low by alerting operators to engine and pollution control equipment problems. They also aid in the diagnosis and repair of complex equipment issues. Vehicle operators should heed all warning lights and refer to operating manuals for the proper response. Manufacturer's instructions must be followed when performing repair and maintenance service on OBD systems.

EPA Resources

U.S. EPA -- Office of Transportation and Air Quality Contacts. This contact list contains the names and telephone numbers of U.S. EPA employees who can answer your specific regulatory questions.

On-Board Diagnostics. Resources relating to the final rule: Regulations Requiring On-board Diagnostic Systems on Highway Heavy-duty Engines used in Large Trucks; Revisions to On-board Diagnostic Requirements for Diesel Highway Heavy-duty Engines used in Smaller Trucks (published February 24, 2009).

Heavy-Duty Highway Diesel Program Compliance Help Page - Provides information to assist the regulated community in complying with the Highway Diesel and Nonroad Diesel Rules.

Health Assessment Document for Diesel Exhaust - EPA/600/8-90/057, May 2002 - This assessment examined possible health hazards associated with exposure to diesel engine exhaust.

Truck and Bus Engines Emission Standards - Guidance, regulations, and general information on emission standards for heavy-duty highway diesel engines and vehicles.

Nonroad Engine Emission Standards - General information, reports, and proposals for nonroad engine emission standards (e.g., marine diesel engines).

Vehicle Emission Standards - Provides links to emission and certification standards for automobiles, motorcycles, trucks, and buses.