A federal judge in San Francisco has given Volkswagen one more month to meet a demand from U.S. regulators for a fix to its diesel engines.

The automaker faced a deadline Thursday to create a fix acceptable to the California Clean Air Authority and the Environmental Protection Agency for diesel engines that emit more than acceptable levels of nitrogen oxides.

U.S. District Judge Charles Breyer said VW could face a trial unless it has a plan by April 21 to recall and repair the 600,000 diesel VW cars in the U.S.

Last September, VW admitted it had installed a cheat device in diesel engines, a software program that ensures the engines meet environmental standards during testing conditions, but spews polluting emissions in real-world driving. The vehicles, with 2.0-litre and 3.0-litre engines, emit up to 40 times the legal limit of nitrogen oxide pollutants.

It presented a proposal to fix U.S. cars earlier this year, but that didn't meet the standards of either the California or federal clean air regulator.

Threat of trial

Judge Breyer said he granted the extension in the case, because he was satisfied that VW had made progress toward a plan to repair the vehicles.

But he warned the German automaker that if it does not meet the next deadline, including explaining which vehicles would be repaired by which dates, he would schedule a trial for this summer.

Breyer is set to adjudicate more than 500 class action suits for fraud and loss of value in the cars by VW owners. He also could rule in the trial over environmental violations pressed by the EPA and U.S. department of justice.

In devising a fix, Volkswagen must consider both environmental impact and any adverse effect the repair may have on fuel economy and driveability of VW diesel cars.

Owners have been waiting eight months for a recall proposal and their cars are unsalable in the meantime.