Motorists at risk of MOT fail with little-known issue possible on petrol and diesel cars
Motorists run the risk of failing their annual MOT test and being unable to use the roads due to a common emissions test problem.
Petrol and diesel owners could fail their MOT test due to a little-known emissions fault which could catch drivers unawares.
Leading mechanic Craig, an expert at Walsall Wood Tyre And Service, stressed emissions failures were not always caused by issues with the catalytic converters.
Instead, a common problem is damage to exhausts within a "small blowhole" likely to cause havoc for emissions readings.
These checks are a vital part of the DVSA MOT check and a reading considered too high will not be awarded a pass certificate.
This would be a slam-dunk MOT failure with motorists having to get their vehicle retested before they can drive on the roads.
Speaking on TikTok, Craig said: "So what’s the most common cause of emissions failures I hear you ask?
"A lot of people are going to say it's the cat, it's the cat, it's the cat (catalytic converter). It’s not. It's actually very very rarely the cat itself.
"We've got a perfect example here. This is a nice 20-year-old car with a rotten heatshield around the car.
"But there is absolutely nothing wrong with that car. What it does however have further back on the exhaust is a leak in the exhaust system itself, a small blowhole.
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"Now what this does, this allows oxygen to enter the exhaust pipe before it comes out the tail where it’s being analysed by the gas analyser.
"At this point, you’re going to find the reading has gone quite high, out the specification, and the vehicle is going to fail."
Motoring experts at KwikFit revealed that the MOT emissions test requires specialist equipment found in garages.
This is used to measure the gases produced by an exhaust compared to official guidelines.
Four gases are measured in the check with oxygen, carbon monoxide, carbon dioxide and hydrocarbon data all collected.
However, Craig stressed hydrocarbons themselves can cause motorists a headache and could provide a major MOT headache.
He added: "The second most common cause is when we fail on hydrocarbons and this is usually down to a misfire or a poor running engine itself. Usually going to be down to plugs or coils but always worth checking compression while you’re there."
