Now that the government has reopened after a 35-day shutdown, EPA employees are going back to work facing a backlog.

Toward the end of the shutdown last week, executives at all three major Detroit automakers expressed concern about certification delays that could affect new models they plan to bring to market.

It can now begin addressing that backlog, but any further delays could still threaten to postpone some new models—especially those with new technology designed to make them cleaner or more efficient.

DON'T MISSTrump administration releases proposal to weaken fuel economy, emissions standards

The EPA National Vehicle and Fuel Emissions Laboratory in Ann Arbor, Michigan, has to sign off on every new car and powertrain before it can go on sale in the U.S. As it does, it issues its official EPA fuel economy estimates, which often match automakers' preliminary estimates, but sometimes don't.

Automakers test the cars themselves and send the results to the lab, but the lab retests many models and powertrains to double-check the results before it will certify the new cars for sale.

That's especially true if the cars include any new technology designed to improve fuel economy or reduce emissions. These days, in light of tightening emissions and fuel-economy standards, that includes most new models.

READ THIS: 2020 Ford Explorer Hybrid signals the start of a big electrified push

With the government shut down for 35 days, needed tests didn't get done, which could delay certifications.

"Like other automakers, we are currently awaiting decisions in the certification process for a few model year 2019 and 2020 vehicles," GM spokeswoman Jeannine Ginivan told Reuters last week during the shutdown. She said the automaker had not had to postpone launching any new vehicles yet, as of last Friday. 

Fiat Chrysler CEO Mike Manly said at the Detroit auto show early this month that certification delays had affected the launch schedule of his company's new heavy-duty Ram 3500 pickups, but did not list any other models affected.

CHECK OUT: Ford hybrid police cars are pursuit-rated, ready for NYPD action

Ford told Reuters last week during the shutdown that EPA certification delays had not affected any of its upcoming models yet, but that if the shutdown continued for another 30 days, it would be likely to delay the launches of some new vehicles. That could include the new Explorer Hybrid, for example. The EPA has not yet published official fuel-economy estimates for the Explorer Hybrid.

Now that Congress and the President have reached a deal to reopen the government temporarily, through February 15, the EPA lab can start testing again and begin to whittle down whatever backlog has developed.

President Trump, however, has threatened to shut the government down again then if any new funding resolution from Capitol Hill still doesn't include funding for a border wall. If EPA testers haven't chugged through their backlog from the first shutdown by then, it could begin to delay more vehicle launches—especially of the cleanest and most fuel efficient versions of new cars.

Electric cars are less likely to be affected, because, while the EPA still has to certify their efficiency, they have no tailpipes to test.