The EPA’s pledge to conduct more rigorous emissions testing to ferret out offenders such as Volkswagen’s illegally rigged diesel engines is forcing automakers to bake in an extra four to six weeks so vehicles can clear the certification process on their way to showrooms, a Fiat Chrysler executive says.

Within the past month, the federal agency has alerted automakers of the backlog in certifying every automaker’s ’16 model, says Bob Lee, FCA’s vice president-engine and electrified propulsion systems engineering.

“The letter said, ‘Please be aware that some undetermined amount of time longer will be required to approve your request for certification,’” he tells WardsAuto editors during a recent interview.

Lee estimates ’16 vehicles with carryover engines will need an extra four weeks for certification, while models with all-new powertrains could need six weeks or more to clear the process. Still, he knows it’s only a guess. “We don’t have any idea if that’s enough (time) or not.”

For the U.S. market, automakers conduct their own emissions tests and submit findings to the EPA’s test lab in Ann Arbor, MI, which reviews data and grants certification, denies it or requests additional information.

“There’s about a one-in-four chance they will say, ‘Give us that vehicle. We want to test it,’” Lee says. “Then it goes to Ann Arbor, where it will go through the full test sequence there.” Every Fiat Chrysler vehicle called in for further scrutiny has passed, he says.

With about 400 light vehicles available each year in the U.S., the EPA does not have capacity to test every one of them.

In the case of Volkswagen, the German automaker admits some of its software engineers included code that identified when emissions were being tested on diesel vehicles. If so, equipment designed to scrub oxides of nitrogen emissions were functioning properly.

But when vehicles were being driven by real-world customers, away from test benches, the emissions controls shut off, resulting in better performance and fuel economy but exceeding tailpipe limits by a wide margin.

VW has recalled some 11 million vehicles worldwide, including a half-million in the U.S., with plans to fix the vehicles.