Media playback is unsupported on your device Media caption John German talks to Wake Up to Money's Adam Parsons and David Jones

"I'm just a simple engineer from Michigan," says John German, the man who helped discover the Volkswagen emissions scandal.

Speaking to Wake Up To Money on BBC Radio 5 live, Mr German revealed he had told VW about his findings in May 2014, but the company had failed to fix the problem.

He now thinks cars produced by other brands should be investigated for the emissions-cheating "defeat device" software found in some VWs.

John German is the US co-lead of the International Council on Clean Transportation (ICCT), a not-for-profit organisation dedicated to reducing vehicle emissions.

He told Wake Up To Money the story of how they got started with their investigation.

"There's been a consistent stream of data suggesting that diesel cars in Europe have high NOx (nitrogen oxides and dioxides) emission," he says.

"So we had this bright idea - let's look at US where the emissions standards are more stringent.

We thought the cars there would be clean, and we could take the results to Europe and say, 'hey look, they're clean in the US, why can't you do it in Europe?'"

'Something wrong'

On 20 September, VW's then chief executive, Martin Winterkorn, issued a public apology, admitting to "breaking the trust" of customers.

Image copyright John German Image caption The ICCT team initially thought there was something wrong with their equipment

But as Mr German explained, his ICCT organisation had been investigating VW since 2013.

To put it simply, Mr German and his team abandoned standard emissions lab tests and instructed researchers to take their cars - a VW Passat and Jetta, and a BMW X5 - out on the roads to simulate ordinary driving conditions.

What they found - with a machine in the cars' boot and probe down the exhaust pipe - was explosive.

"When [the researchers] first saw it they thought there was something wrong with their equipment," says Mr German.

"We found high - very high - emissions in the real world.

Image copyright John German Image caption Mr German has been much in demand for media interviews

"The Passat had emissions five to 20 times the standard. The Jetta was worse. It was 15-35 times the standard.

"And then when they got to the X5, they were recording very, very low emissions again - so they were like 'OK - it wasn't our equipment'."

The data was clear enough, but Mr German didn't want to pre-judge an enquiry.

"While we suspected it might be a defeat device we never said that, and we turned our data over to the California Air Resources Board (CARB) and Environmental Protection Agency (EPA) and let them do the investigation.

"The words 'defeat device' are not something you ever say unless you're certain."

'Completely inexplicable'

Mr German finished his report and turned over all his team's data to the EPA in May 2014.

He says he also sent a courtesy copy to Volkswagen.

Image copyright John German Image caption Mr German says he wants the cars of other manufacturers to also now be tested

Then in December 2014, the EPA announced VW was issuing a software fix to solve some vehicles' emissions problems.

Mr German says: "[VW] swore up and down to the agencies that this would fix the problem.

"But CARB went out in May [2015] and tested some of the vehicles with the fix and the emissions were still very high.

"VW tried to offer a lot of other explanations for the high emissions for a very long period of time. It wasn't until September 2015 that they finally admitted to the agencies there was a defeat device."

Mr German resists being drawn when asked what action should be taken against VW. "I'm just a simple engineer from Michigan - I don't get into that," he says.

But he is clear on one thing.

"This is the part that I find to be completely inexplicable. VW had a chance to fix it, and yet they continued to try and hide the fact they had a defeat device."

Having discovered the fault in VW, Mr German now thinks wider questions needs to be asked.

"We do not have any data or information that suggests any other manufacturer also is using a defeat device," he says. "But it absolutely needs to be investigated."