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MOT Failures Guide · Emissions

Emissions MOT Failures

Emissions failures affect petrol and diesel vehicles differently. A petrol car fails on CO and HC levels — usually caused by a dead catalytic converter or failed oxygen sensor. A diesel fails on smoke opacity — typically a blocked or removed DPF. Here is exactly how both tests work.

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Emissions MOT Failures

Quick Answer

The MOT emissions test is separate from the mechanical inspection. Petrol vehicles are tested for carbon monoxide and hydrocarbons; diesel vehicles for smoke opacity. Most emissions failures are caused by a degraded catalytic converter, failed lambda sensor, or on diesel vehicles, a blocked or removed DPF. Costs range from £80 (sensor replacement) to £1,500 (DPF replacement).
Two different tests

Petrol vs Diesel Emissions Tests — How They Differ

Petrol Emissions Test

CO (carbon monoxide) measured at idle and fast idle

HC (hydrocarbons) measured at fast idle

Lambda (air-fuel ratio) measured — must be within 0.03 of stoichiometric (1.00)

Pre-1993 vehicles have higher permitted CO limits

OBD (on-board diagnostics) also checked on post-2002 vehicles — no stored emissions faults

Diesel Emissions Test

Smoke opacity measured during a free-rev test

Engine must reach operating temperature before test

Multiple revs are recorded — average must meet standard

DPF presence confirmed — removal is automatic failure

Post-2015: OBD also checked for DPF-related fault codes

What it costs

Common Emissions Failure Causes & Repair Costs

Component / causeFuel typeTypical UK repair costDifficulty
Catalytic converter degraded/failedPetrol£300–£1,200Garage
Lambda/oxygen sensor failurePetrol£80–£250DIY possible
Ignition fault (plugs, coils, leads)Petrol£100–£350DIY possible (plugs)
Mass Air Flow (MAF) sensor faultPetrol/Diesel£60–£200DIY possible
EGR valve stuck or blockedPetrol/Diesel£80–£400Garage
DPF blocked — partial regeneration neededDiesel£80–£200Garage forced regen
DPF removed/replaced with straight pipeDiesel£500–£1,500 (replacement)Garage — illegal to remove
Fuel injectors over-fuellingDiesel£100–£400 (cleaning or replacement)Garage
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Questions & answers

Frequently Asked Questions

What does the MOT emissions test measure?
For petrol vehicles: carbon monoxide (CO) concentration at idle and fast idle, and hydrocarbons (HC). Lambda (air-fuel ratio) is also measured. For diesel vehicles: smoke opacity during a free-rev test (the Hartridge Smoke Unit measurement). The specific limits depend on the vehicle's age and Euro emission standard. Pre-catalyst petrol cars (pre-1993 roughly) have higher permitted limits.
Why did my car fail the emissions test?
Petrol emissions failures are most commonly caused by: faulty oxygen (lambda) sensor causing rich running, degraded or failed catalytic converter, failing ignition components causing misfires, or an engine running on the wrong fuel mixture due to a faulty MAP or MAF sensor. Diesel failures are usually caused by a blocked, damaged, or removed DPF, or injectors that are over-fuelling.
How much does it cost to fix an emissions failure?
Lambda/O2 sensor replacement: £80–£250. Catalytic converter replacement: £300–£1,200 depending on vehicle. Full ignition service (plugs, coils, leads): £100–£350. DPF regeneration service: £80–£200. DPF replacement: £500–£1,500. Fuel injector cleaning: £80–£200. EGR valve cleaning or replacement: £80–£400.
Can I fix an emissions failure by adding fuel additives?
Fuel additives can help in some cases — particularly for diesel DPF regeneration or as a temporary measure for mild catalytic converter contamination. They are not a fix for a failed catalytic converter, broken lambda sensor, or mechanical fuelling fault. If the car has genuinely failed the emissions test, a proper diagnostic is necessary to identify the root cause before spending money on additives.
What is the DPF and why does it fail the MOT?
The Diesel Particulate Filter (DPF) captures soot particles from diesel exhaust. It is standard on diesel cars from approximately 2005 onwards. A blocked DPF causes excessive smoke opacity and will fail the emissions test. A removed DPF (illegally removed) is an automatic failure regardless of smoke output. Regeneration through an extended motorway run or a forced garage regeneration cycle can clear a mild blockage.
Do electric vehicles need an emissions test at their MOT?
No. Battery electric vehicles (BEVs) produce no exhaust emissions and are exempt from the emissions test component of the MOT. They still undergo all other standard MOT checks including brakes, tyres, lights, suspension, and seatbelts. Plug-in hybrids (PHEVs) are tested in their conventional mode as they have a combustion engine.
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SR

Ste Robinson

Founder, MotR · Last reviewed 12 June 2026

Ste Robinson is the founder of MotR, helping UK drivers check MOT history, tax status, vehicle records and ownership information using official DVLA and DVSA data.

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Every emissions test result, advisory, and failure — official DVSA data in 30 seconds.

Emissions MOT standards sourced from the DVSA MOT Inspection Manual and Euro emission standards. Repair costs are UK independent garage estimates, 2026.

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