WASHINGTON As expected, the U.S. Environmental Protection Agency has announced it will require onboard truck and bus emissions diagnostics systems for at least one engine family per diesel engine manufacturer starting next year.

EPAs final OBD rule formally takes effect on April 27.

If a bus or trucks emissions of nitrogen oxides and particulate matter rise to twice the allowable levels mandated by EPAs 2010 standard the driver must be alerted by a warning light.

By 2019, all engines will have to comply with the mandate, and the onboard diagnostic systems will have to alert drivers whenever emissions exceed the 2010 threshold.

"(Onboard diagnostics) is a complex and difficult regulation with which to comply," the EPA said in its announcement. "Should mistakes or errors in regulatory interpretation occur, the ramifications would be limited to only a subset of the new vehicle fleet rather than the entire new vehicle fleet."

Onboard emissions diagnostic systems use electronic sensors to communicate with engine control modules and ensure that emission control technology is functioning properly.

The USEPA final rule follows by about six months the publication of a similar mandate by the California Air Resources Board, which obtained a waiver from EPA in September to create its own on-board diagnostics regulations for heavy-duty diesel engines.

The EPA rule reportedly does not differ significantly from the onboard diagnostic standards set by California.

The apparent reason for the phase in of onboard diagnostic systems for heavy-duty diesel engines is a lack of sophisticated sensors. Getting accurate measurements on the sensors is currently a challenge. Of particular concern to manufacturers is the lack of sensor technology for measuring levels of particulate matter.

The USEPA acknowledged the lack of particulate matter sensor technology in a technical support document released along with the final rule, noting that "sensors which can directly measure exhaust PM will not be available for commercial sale in 2010."

The EPA noted that "certain sensor capabilities have not advanced as we expected" and that tightening emission thresholds too quickly could "lead to more onboard diagnostic detections and more onboard diagnostic-induced repairs and, perhaps, many onboard diagnostic-induced repairs for malfunctions having little impact on emissions."