AMERICANS have never held diesels in high regard and the scandal now engulfing Volkswagen may be the final nail in the coffin.

If you’ve been living in a cave for the past couple of days, the world’s biggest car company (by sales) admitted over the weekend that it had built software into half-a-million diesel cars that allowed them to pass stringent US emissions tests.

The digital dodge came to light after a European investigation last year found that certain diesel engines emitted higher levels of pollutants when subjected to so-called transient loadings (such as being driven up a hill or accelerating).

Testers found that in some instances the average on-the-road emissions levels of nitrogen oxides were seven times the certified European limits.

That’s bad enough, but, according to US legislators, when the tests were repeated in the States some VW diesels may have been emitting as much as 40 times the allowable level of certain pollutants.

According to the International Council on Clean Transportation (ICCT), which carried out the initial investigation, the problem lies in the test procedures.

“Ideally, the driving cycle ... will have been laid out in such a way that they provide a realistic approximation of the actual condition s vehicles encounter on the road,” its report said. “However, this is not always possible because... results from different vehicles [should be] directly comparable, and all vehicles sold in a given market are held to the same standards.

“This has led to vehicles being certified through laboratory procedures that cannot capture the whole range of operating conditions vehicles encounter during real use.”

Volkswagen has been the champion of diesel in the US market.

While other manufacturers have invested in hybrids and electric vehicles VW has championed its ‘clean diesel’ technology.

Last year it sold 78,847 diesel cars in the US. Sister company Audi sold 15,732 TDIs. Between them they held more than 65 per cent of the diesel passenger car market (although Ford sold 118,245 diesels most of them were pickup trucks).

VW has worked hard to convince US drivers that diesel is no longer the dirty alternative (and the ICCT’s tests show that some diesels can meet the toughest tests) – but all its hard work has been rendered meaningless by these revelations.

The company has already suspended sales of affected diesels in the US and ordered an external investigation into the issue.

CEO Martin Winterkorn said: “I personally am deeply sorry that we have broken the trust of our customers and the public.”

Shortly, it will begin recalling hundreds of thousands of affected cars for a fix to remedy the problem.

The nightmare now for VW’s engineers is how to fix the problem without making its cars worse to drive (lowering nitrogen oxide can hit performance and fuel efficiency), especially in a country as litigious as the US. It’s already had one gone (hundreds of thousands of cars were recalled for a software patch last year but when they were retested the nitrogen oxide levels were still too high).

Some experts have speculated that the simplest way to fix the problem would be to fit a urea injection system, but designing then retrofitting such a thing to nearly half-a-million cars would be cripplingly expensive.

Then there are the fines. Theoretically, VW could be fined $37,500 per vehicle, which adds to up an eye-watering $18 billion.

The shame of all this is that VW’s diesels are still great engines. They are easy to drive, relaxing, quiet and very frugal – all the features motorists look for when they buy a new car.

Unfortunately, it now appears they aren’t as clean as we all thought.

Looking ahead, the VW debacle is probably the end for diesel passenger car sales in the US. The cost of repairing the reputational damage of diesel tech is too much.

It may also have implications for Europe. After years of telling us diesels are the environmentally friendly alternative to petrol, Western governments have gone cold on TDIs – and this could be just the excuse they need to crackdown on technology hundreds of thousands of drivers bought into.