WASHINGTON The U.S. Environmental Protection Agency has extended the comment period for a proposed regulation requiring onboard diagnostics that measure heavy-duty bus and truck exhaust emissions.

The extension comes amid a chorus of requests that the EPA give engine manufacturers more time to develop the onboard emissions sensors or loosen the proposed standards for the monitoring devices.

Late last year, the EPA proposed regulations that would require engine manufacturers to phase in onboard electronic measuring devices starting in 2010. Then, in 2013, all bus and truck engines would have to meet the requirement.

The proposed devices are supposed to monitor the more complex control systems that will be needed to meet the stringent 2010 EPA emissions standards. The diagnostic sensors must scan particulate filters, catalytic converters and other emissions-treatment systems for possible malfunctions.

They would store their measurements in the engines computer to assist in diagnosis and repairs.

In January, the EPA published the formal notice of proposed rulemaking, with the comment period open until March 24. Following a request from the Engine Manufacturers Association, the agency has extended the comment period to May 4.

The engine builders say they can meet the requirements of the proposed regulation by 2010, but they want the EPA to change the thresholds at which the new emissions sensors are triggered. They consider the specific limits suggested by the EPA to be too stringent.

However, many in the trucking industry are worried the engine makers wont have enough time to adequately test the effectiveness or accuracy of the sensors, making early 2010 engine buyers guinea pigs for the new gear. The trucking groups are asking that the rule be delayed.