Onboard Diagnostics For Emission Control Systems

The U.S. Environmental Protection Agency (EPA), in December 2008, finalized regulations that require the emissions control systems of large highway diesel and gasoline trucks to be monitored for malfunctions or deteriorations via an onboard diagnostic (OBD) system. Those systems are similar to those that have been required on passenger cars since the mid-1990s. These EPA regulations, being phased in over the next several years, require OBD systems on 2010 and later heavy duty engines used in highway applications over 14,000 pounds. The rules also make changes to certain existing OBD requirements for smaller highway heavy duty diesel trucks.


For these highway applications over 14,000 pounds (Class 4 vehicles), EPA is requiring that one engine family per manufacturer be certified to the OBD requirements through 2012 model years. Beginning next year, 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, say EPA officials, and to provide the industry with a learning period prior to implementing the complex OBD requirements on 100 percent of their highway product line.

The EPA projects that the OBD requirements will result in an increased hardware cost of roughly $60 per diesel engine and $70 per gasoline engine used in applications over 14,000 pounds.

The agency anticipates that the new requirements for diesel heavy duty applications under 14,000 pounds will have no increased hardware cost since these engines and vehicles have complied with OBD requirements since 2004.

EPA officials say the new OBD regulations should have no impact on fleet operations other than to provide better diagnostic and repair information, which will assist vehicle technicians in making proper emission control system repairs.


The rules require manufacturers to install OBD systems that monitor the functioning of emission control components (diesel particulate filters, oxides of nitrogen [NOx] reducing catalysts, etc.) for any malfunction or deterioration that could cause the exceeding of a set of emissions thresholds.

Furthermore, EPA is also requiring that all emission-related electronic sensors and actuators be monitored for proper operation.

Beyond this, the regulations also require driver notification, via a dashboard light or malfunction indicator light, when the diagnostic system detects an emission-related problem. These lights are commonly referred to as the “Check Engine” light.

When a malfunction occurs, diagnostic information must be stored in the engine’s computer to assist in diagnosis and repair of the malfunction, the EPA regulations state.

Additionally, EPA is requiring manufacturers 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. This information includes, but is not limited to, manuals, technical service bulletins, a general description of the operation of each OBD monitor and more.

All of the OBD requirements will help to ensure that “the significant benefits of EPA’s 2007 and 2010 heavy duty highway standards will be realized in-use,” say EPA officials.


The EPA considers OBD to be a critical element to an overall emissions control program.

In January 2001, EPA established a comprehensive national control program - called Clean Diesel Trucks and Buses - to regulate the heavy duty vehicle and its fuel as a single system. As part of this program, new emission standards for heavy duty highway engines and vehicles began to take effect in model year 2007 and were phased in through 2010.

The emissions reductions associated with Clean Diesel Trucks and Buses are estimated to result in more than $70 billion in benefits through reduced hospitalizations and lost work days, EPA officials note. The OBD requirements will help to ensure that these benefits are realized.


For 2010 and later model year highway heavy duty diesel applications under 14,000 pounds, the EPA set a new emissions threshold for monitoring of the diesel particulate filter to detect a catastrophic failure of the device.

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 2004 NOx standard was much higher than today’s very low level set with the 2010 NOx standard, explain EPA officials.

Heavy duty engines and, in particular, diesel engines, tend to have very long, useful lives, the officials point out. With age comes deterioration and a tendency toward increasing emissions.

With the OBD systems EPA is requiring, the agency expects that heavy duty engines “will continue to be properly maintained and, therefore, will continue to emit at low emissions levels, even after accumulating hundreds of thousands and even a million miles,” say EPA officials.

California, through its Air Resources Board, was the first state to establish and implement heavy duty onboard diagnostic requirements. For its final OBD regulations, EPA worked closely with California to develop a consistent set of national OBD requirements.


Emissions control of highway engines and vehicles depends on properly operating emissions control systems, EPA officials say. However, to realize significant emission reductions and health benefits, the emission control systems on heavy duty highway engines and vehicles must continue to provide 90 to 95 percent emission control effectiveness throughout their operating life.

The new EPA OBD requirements, in conjunction with and support of EPA’s existing compliance programs, “will help to ensure that emission control systems continue to operate properly by detecting when those systems malfunction, by then notifying the driver that a problem exists that requires service and, lastly, by informing the service technician what the problem is so that it can be properly repaired,” they conclude.