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...


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.

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