Navistar is suing to force the Environmental Protection Agency to go back and re-test SCR engines for compliance with clean air standards and other aspects of the 2010 emission rule.

In the suit, filed July 5 at the U.S. District Court for the District of Columbia, Navistar contends that EPA failed to comply with requirements for certification testing in clearing selective catalytic reduction systems for production and sale.

"The reason behind the suit is to seek a level playing field," said Navistar spokesman Steve Schrier.

Navistar is the only truck manufacturer that does not use SCR technology to meet the 2010 standard, and this is the latest in a series of challenges it has brought against the rule.

The company contends that drivers can get around SCR systems. In a test conducted by a contractor Navistar hired, drivers operated SCR trucks for extended periods without DEF, ran engines with water rather than DEF and manipulated DEF warning systems.

EPA sidelined these contentions in a guidance issued last month, saying that it "does not believe Navistar's findings reflect the overall efficacy of SCR systems on heavy-duty diesel engines currently in operation or the way they are actually used."

In that guidance the agency also proposed to tighten the standard. It called for warning systems to let drivers know when they are running low on diesel exhaust fluid, for instance. It also proposed that manufacturers make the truck down to 5 mph if the DEF tank runs down to empty or the SCR dosing is not working, and that they come up with a way to identify and respond to poor-quality DEF.

EPA now has until the end of August to file a reply or a motion to dismiss the suit. Other engine manufacturers have the option to intervene.

At this point it is not clear what would happen if the court granted Navistar's plea to re-do the tests. Schrier said he could not comment on the possibility that such a ruling might force SCR engine manufacturers to halt production and sale pending completion of the tests.