A tough new MOT testing system for 30m cars on Britain’s roads starts on 20 May – threatening expensive bills for many diesel drivers and chaos at MOT centres as fears grow that new emissions testing equipment will not be ready on time.

The old-style “pass or fail” test is being replaced with a new MOT system that will classify faults as minor, major and dangerous, with the last two resulting in an automatic failure. Crucially, new diesel emission checks will be required to make sure vehicles meet “plate standards”. If they fail, the repair bill could run into thousands of pounds, which for some vehicles will be more than the car is worth. Most at risk are drivers of any diesel car from around 2006 onwards – models that came with a diesel particulate filter (DPF) to clean up its emissions.

Many unscrupulous garages have removed DPFs because they are expensive to replace when they fail, or simply because the owner wanted to improve the car’s performance, despite the environmental damage caused as a result. But MOT testers are now under orders to fail any car that has had the filter removed or tampered with. Those same cars will also be failed if they produce any “visible smoke of any colour”, which could lead to many failures.

The cost of reinstating a DPF or replacing a broken one will, in many cases, be more than the car’s value meaning it is likely to be scrapped.

Bizarrely, older, often more polluting, diesel cars – those built before 2005-06 when DPFs were first introduced – will see no change, and will be emissions-tested as they are now.

The new system creates the potential for confusion as testers will have to judge if faults are dangerous, major or minor

The tougher emissions check is just one of a range of new measures that industry experts say will result in many more cars being scrapped than currently happens. The tests are being introduced to meet a new EU roadworthiness directive. Similar measures are being introduced across the EU.

Bumper damage, tyre pressures, reversing lights and even headlight washers will be more rigorously tested. Another significant change occurs if a car fails its MOT for something considered dangerous. In that case, the owner risks a £2,500 fine and six penalty points if they drive away from the testing station even if the car’s MOT still has a few days to run.

It means dangerous failures will in effect have to be rectified by the testing garage – with some mis-selling likely as a result – or be towed to another garage, with the extra expense that would entail.

Testers are expecting a rush of work in the coming weeks as drivers wake up to the changes and decide to get one last year out of their older diesel car. Alongside the new emissions check, a number of other changes are being introduced:

• All vehicles will be checked for fluid leaks, which pose an environmental risk. A steering box leaking oil would be a minor fault. If it is dripping, that would be a major defect and a fail. The brake fluid will be checked for contamination.

• Vehicles first registered from 1 September 2009 will have to have working reversing lights and headlight washers, if installed. The latter will catch out many motorists.

• Tyres will be examined for obvious under-inflation.

• New checks will be made on prop shafts, rear-drive shafts and the security and condition of bumpers.

After 20 May, cars with minor defects will still pass, but faults will be recorded on the MOT certificate, in the same way as an advisory notice now. A tyre that was worn but still legal would be considered a minor defect.

The RAC says it fears many motorists could end up being confused by the new categories, which give an indication about the seriousness of defects identified in an MOT.

“Rather than MOT failures being black and white, the new system creates the potential for confusion as testers will have to make a judgment on whether faults are dangerous, major or minor. This will surely be open to interpretation, which may lead to greater inconsistency from one test centre to another,” says RAC spokesman Simon Williams.

“Drivers who have properly maintained their diesel vehicles shouldn’t have anything to fear from the new MOT rules for diesel car emissions. But replacing a DPF tends to be expensive so some motorists will be in for a nasty surprise if their vehicle fails the MOT due to a worn-out DPF.”

He adds: “A number of motorists will also get a shock when they discover that their recently acquired secondhand diesel car has had its DPF tampered with or, worse still, removed.”

Dan Powell, editor of car website HonestJohn.co.uk, says the crackdown on DPF removals is long overdue. “MOT testers have told us that the old visual inspection was unfit for purpose as it made it near impossible to identify tampering or removal. This gave a free pass to owners who were willing to put lives at risk to save a few hundred pounds on their maintenance bills by having the DPF removed.”

He added that another important update was the banning of aftermarket high intensity discharge headlights, which often blind drivers going the other way as they are poorly installed.

Gareth Llewellyn, chief executive of the Driver and Vehicle Standards Agency (DVSA), says: “I would urge all motorists to familiarise themselves with the new items that will be included in the test so they can avoid their vehicle failing its MOT. To be safe and responsible, motorists should also carry out simple checks on their vehicles all year round.”

The DVSA has confirmed that cars will still be required to have their first MOT on their third birthday. The government had consulted on whether cars might not need to be tested until their fourth birthday, but decided to maintain the three-year rule, with cars required to be tested annually thereafter, rather than every two years as was also proposed. The rules are different in Northern Ireland, with MOTs due after a car’s fourth birthday.