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A programme to double-check if older cars are breaking pollution laws was dropped by the government five years ago, the BBC can reveal.

The Department for Transport used to take random cars off the road and then test the levels of dangerous gases coming from the exhaust pipe.

But a Freedom of Information request for the BBC shows that the spot-check scheme was dropped in 2011.

That left the carmakers to police themselves.

Questions

The chair of the Transport Select Committee, Louise Ellman, has called the news "deeply disturbing".

The committee is currently investigating vehicle emissions after VW admitted cheating pollution tests in the US.

"I will be pursuing both the VCA (Vehicle Certification Agency) and the Department for Transport on how this happened when they appear in front of the committee as part of our ongoing enquiry," said Ms Ellman.

However, the government has not broken any rules.

EU regulations only oblige carmakers to test their own vehicles. Manufacturers hand their results over to the relevant type-approval authority, but it would appear no-one independent sits in the lab, double-checking what's going on.

Who else double-checks?

Although the UK government has stopped double-checking ordinary cars, some other governments still do it.

The Swedes and the Germans both have a programme. The Americans also spot-check cars based on a number of criteria. Sometimes they pick a model at random. They also select vehicles based on the manufacturer's data and on inquiries and complaints from the public.

John German is one of the key people who helped catch VW cheating. He is from a group called the International Council on Clean Transportation (ICCT) and he told me that the latest available figures show spot-checks had some effect in the US.

"Based upon this in-use testing, as well as defect reports, there were 42 voluntary and mandatory emission-related recalls in 2009, 39 in 2010 and 53 in 2011," he said.

Meanwhile, his colleague Peter Mock said he was "convinced that it is very important for governments to carry out vehicle tests".

"Even though of course the testing itself is only one part of it," he adds. "The other part is to ensure that the vehicles chosen for testing are representative and not specially prepared, that the testing is carried out under realistic conditions, and that anything suspicious found would trigger thorough investigations and ultimately also penalties."

'Divert resources'

The UK government argues that it unearthed only two problems in a decade, and that taxpayers' cash was better spent elsewhere.

"Manufacturers have always been required to conduct 'in-service' testing to ensure their vehicles continue to meet regulations," a spokesman told the BBC.

"The Vehicle Certification Agency (VCA) previously conducted additional testing on top of this, but a decision was taken in 2011 to divert resources to improving our understanding of the effect after-market alterations are having on vehicle emissions."

In other words, they stopped checking normal road cars and moved to an important, but more specialist area, vehicles that had been "chipped" by customers. That's where the software is changed to improve performance.

They changed tack and they also cut the budget. It used to be about £200,000 a year. Last year, they spent just £42,300.

"We are fully committed to improving air quality and have led calls at a European level for Real Driving Emissions testing that will mean all new cars have to adhere to robust real-world standards from as early as next year," the government spokesman added.

So, what is 'in-service' testing?

The rules say that older cars must still pass an official, European emissions lab test. It's there to protect public health, because scientists estimate that pollutants from cars kill tens of thousands of British people each year.

Manufacturers have to double-check their own vehicles and pass on the results to the government, whose experts can order more tests if they suspect a problem.

Pollutants

Since 2005, government inspectors tested around 10 different models per year. Details seen by the BBC show they checked a number of VW and Skoda diesels in that time.

Some of those cars could potentially have been fitted with a defeat device, although don't forget it was well hidden in the computer, and inspectors were not allowed to look at computer software.

Between 2005-06 and 2010-11, 227 individual tests were completed (76 petrol and 151 diesel) and 87 vehicles failed to achieve a "pass" for all pollutants (14 petrol and 73 diesel). When cars failed, they carried out more tests.

Overall, the government says there were just two model failures in a decade, a Mitsubishi Carisma Petrol, (2005-06 test) and a BMW Mini One D (2008-09 test).

In the case of the BMW Mini, the government says that the German approval authority (KBA) was told "to enable them to take action". It does not say what that action was, if any.

'No further action'

And with the Mitsubishi, the manufacturer was contacted directly and they found "a number of anomalies in the test vehicle that could account for the failed result".

After further talks with the manufacturer, "no further action was taken".

In a decade, the Driver and Vehicle Standards Agency has never taken action against a car maker because of "in-service" testing.

Testing has re-started again.

After the VW scandal, the UK government actually began car spot-checks again. The BBC was invited to see them in action.

But it's a one-off programme and it will stop in the spring.