The MOT Emissions Test Explained: Petrol, Diesel and DPF Rules

Emissions testing is the part of the MOT people understand least and worry about most, largely because the result arrives as a number on a printout with no explanation attached. It is also the area where the rules have tightened most sharply in recent years, particularly for diesels.

The underlying logic is simple. The tester puts a probe in the exhaust and measures what is coming out. If the readings exceed the limits that apply to that specific vehicle, it is a major defect and the car fails. What complicates it is that the limits depend on fuel type, the date the vehicle was first used, and whether it was built with a particulate filter — and that for diesels there is a separate rule that has nothing to do with the meter at all.

What the emissions test is actually checking

The MOT emissions test has two halves: a visual inspection of the exhaust and emissions control system, and a metered measurement of the gases.

The visual half comes first and catches more cars than owners expect. The tester checks that the exhaust system is complete, secure and free from serious leaks, and that it effectively silences the vehicle. They check that a catalytic converter is present if the vehicle was originally fitted with one, and that a diesel particulate filter is present if it was originally fitted with one. Since 2018 they also check for emissions control equipment that is obviously missing or defective — an absent EGR valve, a disconnected selective catalytic reduction system on a vehicle that uses AdBlue, or a filter that has clearly been hollowed out.

A leak matters for two reasons. It is a defect in its own right, and it also makes the metered test meaningless, because air drawn in through the hole dilutes the sample. A car with a blowing exhaust joint can produce a nonsensical reading that has nothing to do with how the engine is running. The exhaust leak entry in the failure library covers what counts as serious.

Electric vehicles have no exhaust emissions test at all. Hybrids are tested according to the type of combustion engine fitted.

How petrol cars are tested

For a petrol car the tester warms the engine to normal operating temperature, then takes readings with a gas analyser at fast idle — typically around 2,500 to 3,000rpm — and at natural idle. Three values matter.

Carbon monoxide (CO) is a product of incomplete combustion. A high reading generally means the engine is running rich, or the catalytic converter is not doing its job.

Hydrocarbons (HC), measured in parts per million, are unburnt fuel. High HC points to a misfire, ignition problems, or a failing catalyst.

Lambda is the ratio of the actual air-fuel mixture to the ideal one. A lambda of exactly 1.00 means the mixture is chemically perfect. Readings must sit within a narrow band, and a lambda outside it tells the tester the fuelling is wrong even where CO and HC happen to fall within limits.

For most petrol cars first used on or after 1 August 1992, the default limits are 0.3% CO and 200ppm HC at fast idle, with lambda between 0.97 and 1.03, and 0.5% CO at natural idle. Where the manufacturer specified different figures for the vehicle, those can apply instead. Older petrol cars first used before that date and not fitted with a catalytic converter face a much less demanding basic test, with limits of 4.5% CO and 1,200ppm HC.

What usually causes a petrol emissions failure

In rough order of frequency:

• A failing or dead catalytic converter. The single most common cause of a genuine petrol emissions failure on an older car.
• A faulty lambda (oxygen) sensor. The engine management system relies on it to trim fuelling. A lazy sensor produces a rich mixture and a high CO reading.
• Ignition faults. Worn spark plugs, failing coil packs or old leads produce a misfire, which sends HC up sharply.
• A restricted air filter or a fuelling fault, both of which push the mixture rich.
• Simple neglect. Degraded engine oil releases hydrocarbons through the crankcase ventilation system, which the analyser sees.
• A cold engine. A catalyst that has not reached operating temperature cannot convert anything.

How diesel cars are tested

Diesels are not tested with a gas analyser. They are tested for smoke opacity, measured with a meter that shines light through the exhaust plume and records how much of it is blocked. The result is expressed in m⁻¹.

The procedure is a free acceleration test. With the engine at normal operating temperature, the tester checks that the engine appears sound enough to be tested, then accelerates it sharply to the governor cut-off point and releases. This is repeated — normally three times — and the readings are averaged. If the tester believes the test could damage the engine, or the owner or presenter warns of a condition that makes it unsafe, the tester can decline to carry out the metered test, in which case the vehicle is refused.

The smoke limits depend on the vehicle:

The visible smoke rule

This is the rule that has caught out more diesel owners than any other change to the test in the last decade, and it is worth stating precisely.

If a diesel was fitted with a diesel particulate filter from the factory, any visible smoke of any colour from the exhaust during the test is a major defect. Not excessive smoke. Not black smoke. Any visible smoke, of any colour, including white or blue.

The logic is that a properly functioning particulate filter should produce a plume you cannot see. Visible smoke therefore indicates that either the filter is not working or something upstream is badly wrong — and the meter reading is beside the point. A car can record an opacity figure inside the limit and still fail on this rule alone.

Tampering with a particulate filter

A vehicle originally fitted with a particulate filter must still have one to pass. If the tester finds obvious evidence that the filter has been removed, cut open, hollowed out or replaced with a straight pipe, they will refuse to test the vehicle rather than simply fail it. Where the filter housing has been opened and re-welded, the tester is entitled to ask for evidence that the filter core is still present and doing its job.

This matters when buying a used diesel, because "DPF delete" work is still sold and is usually undisclosed. A car with a suspiciously clean recent MOT and a history of earlier emissions problems is worth scrutinising, as covered in red flags in an MOT record.

Why results come out borderline

An emissions figure is not a fixed property of a car. It is a measurement taken under particular conditions, and several of those conditions are within your control.

Engine temperature. The largest single factor. A catalytic converter needs to be hot to work, and a diesel particulate filter only regenerates under sustained load. A car driven two miles from cold to the test centre is being measured at its worst.

Recent driving pattern. A diesel used exclusively for short urban journeys accumulates soot in the filter that it never gets the chance to burn off. The filter fills, regeneration attempts fail, and the smoke reading rises. This is the mechanism behind most diesel emissions failures on cars that are otherwise healthy.

Service condition. Old oil, worn plugs, a clogged air filter and a tired fuel filter all move readings in the wrong direction. None of them are faults exactly, but together they can be the difference between 0.28% and 0.34% CO.

Fuel level and quality. A nearly empty tank of fuel that has been sitting for months does not help.

The state of the exhaust upstream of the probe. A small leak dilutes the sample unpredictably.

What to do in the week before the test

If your car has previously recorded a marginal emissions result, four things are worth doing, in ascending order of cost:

1. Drive it properly before the test. Twenty to thirty minutes at motorway speed, and for a diesel, a sustained period in a lower gear at higher revs to encourage a regeneration.
2. Arrive with the engine hot. Not "warmed up on the drive" — genuinely up to temperature from driving.
3. Get it serviced first, not after. Fresh oil, air filter and spark plugs where applicable.
4. Have the fault diagnosed if the readings were significantly out rather than marginal. A guess at a catalytic converter is an expensive guess.

What an emissions failure costs to fix

Costs vary widely by vehicle, and anyone quoting a single figure is guessing. What follows are the ranges garage quotes commonly fall into, in ascending order — treat them as an orientation, not a price list, and note that parts prices on premium and complex cars can sit well above them.

Two points follow from that table. First, diagnosis is cheap relative to the parts, so paying for a proper diagnostic before authorising work is almost always the economical choice. Second, on an older diesel a failed particulate filter can genuinely exceed the value of the car, which is why filter health deserves attention when buying — see checking a used car before you buy.

Emissions failures and what you may drive

An emissions result above the applicable limit is a major defect. So is a missing catalytic converter, a missing particulate filter on a vehicle that should have one, visible smoke from a DPF-equipped diesel, or an exhaust that leaks seriously or fails to silence the vehicle effectively.

Because these are major rather than dangerous defects, the ordinary rules apply. If your existing MOT certificate has not yet expired, it remains valid and you may continue to drive the car while you arrange repairs. If it has expired, you may drive only to a pre-arranged repair appointment or a pre-booked test. The distinction is set out in more detail in dangerous, major and minor defects explained.

Retesting follows the standard rules. Leave the car at the test centre for the repair and the partial retest is free. Return within 10 working days and a partial retest at a reduced fee may be available. Beyond that you pay for a full test, capped at £54.85 for a class 4 vehicle.

The MOT is not a clean air zone test

One persistent confusion is worth clearing up. Passing the MOT emissions test does not mean your car is exempt from charges in London's ultra low emission zone or in a clean air zone elsewhere in the UK.

Those schemes are based on the Euro emissions standard the vehicle was certified to when it was built — broadly Euro 4 for petrol and Euro 6 for diesel in the ULEZ — not on any measurement of what the car emits today. A well-maintained Euro 3 petrol car that sails through its MOT is still chargeable. A Euro 6 diesel that scraped through on smoke is not.

Nor does the MOT measure nitrogen oxides, which are the pollutant those schemes are principally designed to address. The MOT measures CO, HC and lambda on petrol and smoke opacity on diesel. NOx does not appear anywhere in the test.

The short version

Petrol cars are measured for carbon monoxide, hydrocarbons and lambda at fast idle, with limits that depend on age and specification. Diesel cars are measured for smoke opacity under free acceleration, with a tighter limit where a particulate filter is fitted, plus an absolute rule that any visible smoke of any colour from a factory-DPF diesel is a failure. The visual inspection of the exhaust and emissions control equipment happens alongside both.

Most marginal failures are the product of a cold engine, a car driven only on short journeys, and deferred servicing. Most expensive failures are a catalytic converter or a particulate filter reaching the end of its life. The gap between those two outcomes is largely a matter of how the car is used and whether earlier warnings were acted on — which is why the emissions section of your own vehicle's history is worth reading before you book, and why it is one of the first things to check on a used diesel. You can pull any registration's full record on the MOT history check.