Thousands of motorists are being ripped off when they get their cars serviced, according to an investigation by WhatCar?.

The results reveal that buyers could be better off sticking to local garages than paying top-dollar at swanky manufacturer-approved main dealers.

What Car? conducted a nationwide undercover investigation, using qualified vehicle inspectors in locations across the UK to see which type of garages performed best in tests. Examiners turned up at garages with cars that had been doctored with seven deliberate defects – to see if the mechanics could identify and fix the faults. The results:

Independent garages

Percentage of faults missed: 7%

Independent, often family-run, garages did best in our investigation, spotting 93% of deliberate faults on test cars. Seven of the 10 independent garages we visited identified and fixed every single defect.

Franchised main dealers

Percentage of faults missed: 17%

Main dealers are branded garages representing manufacturers (eg Ford, Toyota). Despite usually being more expensive than independents, the standard of service was worse, says What Car?. They spotted only 83% of deliberate faults – missing 17% of defects. One Toyota dealer in Edinburgh (Western Toyota, Whitecraig) even failed to spot the low, and potentially dangerous, brake fluid level.

Fast-fit garages

Percentage of faults missed: 21%

They missed more than a fifth – 21% – of the faults. More than half the outlets under scrutiny failed to spot at least one fault. One AA Centre in Scotland returned the car with every single defect still present, after agreeing to service the car but then performing (and billing for) a simple oil change.

What Car? magazine editor David Motton said: ‘We were shocked by our findings. When your car is serviced, you should expect that the job will be done to a high standard. This is not happening. We’re also very disappointed that the Government-backed CarWise scheme has fallen by the wayside. If you buy a car on credit, the finance company must be licensed with the Government – so why shouldn’t garages be properly regulated? Getting your car serviced properly can make the difference between driving a safe car and one that is more likely to be involved in an accident.’

What Car? sent a team of qualified vehicle inspectors to 30 garages around the UK, testing a random sample of 10 main franchised dealers, 10 independent garages and 10 fast-fit outlets by asking for a ‘full service’ to be carried out. Every car was expertly prepared with a list of seven standard faults: low brake fluid level, low coolant level, empty screenwash reservoir, misaligned front washer jets, low nearside rear tyre pressure, faulty nearside tail lamp, spare tyre deflated. Each car was inspected before and after every visit to see if the deliberate defects were spotted.