The US environmental regulator said Friday that it will test all diesel car models for pollution "defeat devices" in the wake of the scandal over Volkswagen's vehicles.

The Environmental Protection Agency said it would test cars under actual road conditions rather than just newly produced cars in the lab, after Volkswagen vehicles were shown to have software that overrides pollution controls once they are in use.

Christopher Grundler, Director of EPA Office of Transportation & Air Quality, said the agency would partner with the official Environment Canada agency to more rigorously test diesel cars.

"Today we are putting vehicle manufacturers on notice that our testing is going to include additional evaluation and tests designed to look for potential defeat devices."

"We're not going to tell them what these tests are, they don't need to know."

"Suffice it to say that the smart engineers at EPA and California Air Resources Board and Environment Canada have come up with some clever ways to do this."

Grundler said the agency has had portable devices for testing the pollution performance of vehicles on the road for years.

But it had not focused on passenger diesel cars because they account for barely 0.2 percent of all vehicles on the road in the United States. Instead, the agency had concentrated its efforts on commercial trucks "because that's where diesel emissions are."

The problem with the Volkswagens was  discovered by engineers at West Virginia University, using some of the EPA equipment.

The auto industry has been shocked by the Volkswagen scandal, in which the world's largest automaker was shown to have programmed its 2009-2015 four-cylinder diesel cars to perform well in official anti-pollution testing in the lab, but then override the pollution controls for better road performance when out in the real world.

With the use of the defeat device, the car has more power and saves more fuel, but can spew more pollutants into the air, including nitrogen oxides, in amounts much higher than emissions standards.

Volkswagen "very deeply within a hundred million lines of software code had a sophisticated algorithm designed specifically designed to defeat these tests," Grundler said.

The German company is facing a potential $18 billion in EPA fines and has cost the job of chief executive Martin Winterkorn, who resigned Wednesday.

The EPA said there had not been any recall yet of the VW diesel models of the past seven years involved in the scandal, but it would likely come when the company had developed a way to fix the problem.

Meanwhile, is also said that Volkswagen's 2016 diesel models had not yet been approved for the US market.