MOT Emissions Test

Passing the MOT Emissions Test

Passing the MOT test is essential for any driver who wants to keep their car on the road. For
all diesel and petrol car owners, the MOT includes a strict exhaust emission test – and if
your car does not meet emissions standards, it will fail its MOT. Find out how to pass the
MOT emissions test, and why it’s there in the first place.

Your car will pass the MOT emissions test if the following criteria are met:
✅ There’s no visible smoke coming out of the exhaust
✅ The diesel emissions restrictions are not exceeded
✅ Your car’s diesel particulate filter has not been removed or tampered with in any way

Your car will fail the MOT emissions test if:
❌ Smoke is coming out of the exhaust
❌ The warning light on your dashboard for the diesel particulate filter is on
❌ Diesel emissions are over restricted levels
❌ Visible smoke coming out of the exhaust

A smoking exhaust always needs sorting and a little bit of detective work to figure out the
cause. But the colour of the smoke can give us a clue. For example, a continuous stream of
white smoke (that doesn’t go away shortly after starting the engine) could mean that there’s
a coolant leak, or an issue with the head gasket.
Blue smoke usually means burning oil – somewhere in the engine, oil is leaking and being
burned with the fuel, resulting in dirtier emissions. Unchecked, this could lead to some bigger
problems than just failing the MOT emissions test!

What are the diesel emissions restrictions?
It’s complicated – but basically, there are limits on the amounts and kinds of chemicals your
car can put out of the exhaust. These chemicals include:

● Hydrocarbons (HC)
● Nitrogen oxide (NOx)
● Particulate matter (PM)
● Carbon monoxide (CO)

The amount and type of emissions that your car will be subject to restrictions on will depend
on whether you have a petrol or diesel engine, and the type of vehicle you drive.

Diesel particulate filter (DPF) warning light
The DPF warning light is orange. It can look either like an exhaust muffler with dots in it, or
like a puff of smoke blowing through some dots, but it might look different in other makes or
models of car.
The DPF light comes on when the filter is blocked with soot. This is normally resolved by
driving at high speeds (over 40mph) for 10-15 minutes – but if it doesn’t go away, there
could be some damage present.

If this light is on during your MOT test, your car will not pass the MOT emissions test.

How to lower diesel emissions for MOT

Switch to premium fuel

Make sure tyres are always properly inflated

Install a ​​Selective Catalytic Reduction (SCR) system, or try additives

Stick to a regular car service schedule
Why do MOT tests include a strict exhaust emission test?
The answer is simple: to help protect the environment against pollution. It has worked overall
– carbon monoxide, hydrocarbon, nitrogen oxide and particulate matter emissions have all
reduced significantly since restrictions were introduced.

Book your MOT

Book your MOT test in Eastleigh today, with We Only Do MOTs. We promise to provide an
impartial, independent MOT to everyone – and will carry out most minor repairs for free!
Send your message to or call us on 023 8064 7180

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