Why do cars fail the MOT emissions test?
Most emissions failures come from an engine burning fuel poorly, a worn sensor or a tired catalytic converter, and the cause can usually be found.
Cars fail the MOT emissions test because the exhaust contains more pollutants or smoke than the limits allow. The cause is usually an engine that is not burning fuel cleanly, a sensor giving wrong readings, or a catalytic converter or particulate filter that is no longer doing its job.
A failure sheet can look alarming, but the reading tells a garage where to start looking. Here are the common reasons and how to approach them.
What happens in the MOT emissions test?
On a petrol car, the tester measures gases from the tailpipe such as carbon monoxide and hydrocarbons, with the engine warm. On a diesel, the test measures how dense the smoke is during a quick, controlled rev of the engine.
The tester also checks for visible smoke, that the exhaust is in place and, on diesels that were fitted with a particulate filter, that the filter has not been removed. The exact limits depend on the vehicle and its age, so the official guidance is the place to check.
What are the most common reasons for failing?
The causes differ between petrol and diesel cars, but the common thread is incomplete combustion or a part that treats the exhaust and is worn out.
Having a long overdue service often plays a part too, because an old air filter and dirty oil both affect how the engine runs.
- Misfiring spark plugs, coils or injectors
- A faulty oxygen (lambda) sensor
- A worn or damaged catalytic converter
- A blocked diesel particulate filter or a faulty EGR valve
- An exhaust leak upstream of the sensors, which upsets the readings
- Engine oil burning because of worn seals or rings
What can I do before the test?
Make sure the car is serviced and has no warning lights on. An engine management light normally needs to be investigated first because it can mean a failure by itself.
Check that the oil and coolant levels are right. Give the engine a good run beforehand so it is fully warm. A diesel that mostly does short journeys can benefit from a longer steady drive where the handbook suggests it, but that will not mend a genuine fault.
How does a garage fix an emissions failure?
We begin with a diagnostic scan and read the fault codes and live data, then inspect the ignition, fuel, sensors and exhaust. Finding the root cause matters, because replacing a converter without fixing a misfire will simply cause it to fail again.
The work might be as simple as plugs and a service, or it could involve sensors, valves, a filter or the converter. Cost depends on which part is responsible and on the vehicle, so we would give a quote after looking at the failure sheet and the car.
Can I drive the car after an emissions failure?
If the test result is a failure and the previous MOT has run out, you can only drive it to a pre-booked repair or retest, and only if the car is safe. A failure marked dangerous means it must not be driven at all.
Always check the official rules for your case, and ask us if you are unsure what the failure sheet means.
Emissions failures nearly always have a findable cause, and fixing that cause is better than guessing. Automaster is a garage at Longcombe Cross near Totnes, so ask for a quote or call the quote line.
Common questions
Do I need a retest after repair?
Yes. Depending on where the repair is done and how soon the car returns, a partial retest may be allowed. Check the official retest rules for your situation.
Will an additive or engine cleaner pass the test?
Sometimes it makes a marginal difference, but it will not repair a worn sensor, converter or misfire. A proper diagnosis is a more reliable route.
Is an emissions failure the same on petrol and diesel?
The test and the likely causes are different. Petrol failures often involve ignition, sensors or the converter, while diesel failures more often involve smoke, the filter or the EGR system.
What should I tell you when I ask for a quote?
Send us the make, model and year, the failure reasons from the sheet, and any warning lights. That lets us advise on the likely route before inspecting.
More advice
Ask for a quote
Tell us the make, model, registration and what the car is doing, and we will come back to you. Prefer to talk? Call our quote line on 01803 862152.
Automaster, Longcombe Cross, Longcombe, Totnes TQ9 6PL, United Kingdom