MOT emissions test: petrol and diesel limits explained
How the MOT measures petrol and diesel exhaust, why newer diesels face stricter smoke limits, and why visible smoke from a DPF diesel has failed since 20 May 2018.
Updated
The MOT emissions test measures what comes out of the exhaust. On a petrol car the tester reads carbon monoxide and hydrocarbons with a gas analyser. On a diesel the tester measures smoke opacity, how much light the exhaust smoke blocks. Newer diesels must meet much tighter smoke limits, and since 20 May 2018 a diesel with a particulate filter (DPF) fails if it puts out visible smoke of any colour. For a buyer, the short version is this: a diesel that smokes, or one whose DPF has been removed, is a car to walk away from.
How the emissions test fits into the MOT
Exhaust emissions are part of section 8 of the DVSA inspection manual. The emissions result counts like any other item: a major defect means the car fails.
The test also fails a car whose engine warning lamp is on or does not work, on cars new enough to be covered by that part of the manual (petrol cars first used on or after 1 July 2003 and diesel cars first used on or after 1 July 2008). So a fault code that the seller has "just cleared" may show up again by test day.
Electric cars have no exhaust and no emissions test. The manual also lists hybrid vehicles, including mild hybrids, among those that are not given the metered emissions tests. Our guide to the MOT for electric cars covers what is tested on those cars instead.
Petrol cars: CO, hydrocarbons and lambda
Petrol cars first used on or after 1 August 1975 have their exhaust gas measured. How it is measured depends on the car's age and whether it has a catalytic converter.
- Older cars without a catalyst have their exhaust measured at idle against limits for carbon monoxide (CO) and hydrocarbons (HC).
- Catalyst cars are tested at a fast idle and a normal idle. The tester uses the manufacturer's specified limits where they exist, or the manual's default limits where they do not.
The default limits for the basic emissions test on a catalyst car are:
| Reading | When measured | Default limit |
|---|---|---|
| Carbon monoxide (CO) | Fast idle (2,500 to 3,000 rpm) | 0.2% |
| Hydrocarbons (HC) | Fast idle | 200 ppm |
| Lambda | Fast idle | 0.97 to 1.03 |
| Carbon monoxide (CO) | Normal idle | 0.3% |
Lambda describes the air to fuel mixture. A reading outside the range suggests the engine is running too rich or too lean, often because of a sensor or a tired catalyst.
The tester also looks for smoke. A petrol car fails if it puts out dense blue or clearly visible black smoke for a continuous period of 5 seconds at idle. Excessive dense blue or clearly visible black smoke when revved, enough to obscure the view of other road users, is a dangerous defect. Blue smoke usually means the engine is burning oil.
Diesel cars: smoke opacity
A diesel's exhaust is checked with a smoke meter. The tester revs the engine hard from idle several times and the meter records how dense the smoke is. The result is a light absorption figure, written in m-1. Lower is cleaner.
The limit is the value on the manufacturer's plate, usually a sticker under the bonnet or in a door shut. The tester adds 0.05 to that figure to allow for manufacturing tolerances. Where there is no plate value, the manual sets default limits by the date the car was first used:
| First used | Default smoke limit |
|---|---|
| Before 1 July 2008, non-turbo | 2.5 m-1 |
| Before 1 July 2008, turbo | 3.0 m-1 |
| 1 July 2008 to 31 December 2013 | 1.5 m-1 |
| On or after 1 January 2014 | 0.7 m-1 |
The step from 1.5 to 0.7 is large. A newer diesel has far less room before it fails, which is why a modest amount of smoke can sink a younger car at its test.
DPFs: the rules since 20 May 2018
Most diesels built in the last decade or so have a diesel particulate filter, which traps soot from the exhaust. The MOT changes that came in on 20 May 2018 made two things a major fault:
- Visible smoke of any colour from the exhaust of a car fitted with a DPF.
- Evidence that the DPF has been tampered with.
The manual adds that a DPF which has clearly been cut open and rewelded should be failed, unless the presenter can show a valid reason, such as having the filter cleaned. A car whose DPF has been removed or hollowed out has been tampered with.
Why does this matter beyond the MOT? A removed DPF means the car no longer meets the emissions standard it was built to. Some sellers remove filters to avoid the cost of replacing a blocked one, and the car may run well on the day. You inherit the problem.
A buyer's checklist for diesels
- Ask how the car has been driven. A DPF cleans itself by burning off soot, which needs regular runs at steady speed. A diesel used only for short town trips is more likely to have DPF trouble. Ask the seller directly.
- Watch the exhaust from a cold start. A puff at start-up can be normal on older cars. Persistent smoke, or any visible smoke from a DPF diesel once warm, is not.
- Look at the dashboard. DPF and engine warning lamps should come on with the ignition and then go out. A lamp that never lights may have been disabled.
- Look for signs of cutting. Welds or cut marks on the exhaust around the filter are worth asking about.
- Read the MOT history. Look for emissions fails, repeated advisories for exhaust leaks, or a fail followed by a pass after work. Ask for the invoice that shows what was done.
- Ask for invoices for DPF work. A cleaning or a new filter is a normal repair. Ask what was done and by whom.
If the DPF looks like it has been removed or tampered with, walk away. The car will fail its next MOT and making it legal again means fitting a working filter.
Before you view, our MOT fail rate pages show how often each model fails at each age in Great Britain, and our MOT advisory checker explains common emissions advisories.
Frequently asked questions
What is the MOT smoke limit for a diesel?
It is the value on the manufacturer's plate, plus 0.05 for tolerance. Without a plate value, the default limit is 0.7 m-1 for diesels first used on or after 1 January 2014, and higher for older cars.
Will a diesel without a DPF pass an MOT?
Not if it was built with one. Evidence that the DPF has been tampered with is a major fault and fails the test.
Can a diesel fail the MOT for smoke?
Yes. If it has a DPF, visible smoke of any colour from the exhaust has been a major fault since 20 May 2018. Any diesel can also fail if its smoke reading is over the limit.
Do hybrids have an MOT emissions test?
The inspection manual lists hybrid vehicles, including mild hybrids, among those that are not given the metered emissions tests. The rest of the MOT still applies.
Does an engine warning light fail the MOT?
Yes. An engine warning lamp that is on, or does not work, is a major defect on cars covered by that part of the manual: petrol cars first used on or after 1 July 2003 and diesel cars first used on or after 1 July 2008.