When it comes to MOT testing in the UK, not all vehicles fail for the same reasons. Petrol, diesel, and hybrid engines each have different mechanical components, emission levels, and maintenance requirements, meaning their MOT failure patterns vary significantly. Understanding these differences can help you prepare better, avoid costly failures, and keep your vehicle road-legal.

Why Engine Type Matters in MOT Testing

The MOT test checks the safety, emissions, and roadworthiness of vehicles. While the core test is the same, certain checks vary depending on the fuel type. For example:

• Diesel vehicles face stricter checks for particulate emissions.
• Petrol cars mainly fail due to exhaust gas levels and ignition issues.
• Hybrids may pass emissions easily but fail on battery or electrical safety problems.

Knowing what’s most likely to fail helps owners prepare properly before the test.

Petrol Engine MOT Failures

Petrol vehicles often fail due to:

✔ Emissions issues:

High CO₂ or hydrocarbon emissions are common in older petrol cars, often due to poor fuel mixture or a failing catalytic converter.

✔ Ignition faults:

Worn spark plugs, ignition coils, or timing issues can trigger a fail.

✔ Exhaust leaks:

Rusty or damaged exhaust systems are a frequent cause of failure in ageing petrol cars.

Petrol engines generally perform better in standard emissions checks than diesels, but poorly maintained ones still fail regularly.

Diesel Engine MOT Failures

Diesel vehicles face stricter MOT rules, especially since the introduction of the Diesel Smoke Test and DPF checks.

Common diesel failure reasons include:

✔ Excessive smoke:

Black smoke usually indicates fuel system or injector problems.

✔ DPF (Diesel Particulate Filter) issues:

A missing, damaged, or blocked DPF results in immediate failure.

✔ Emissions tampering:

If a mechanic has removed or bypassed emissions equipment, the vehicle will fail.

Modern diesels require regular long drives to burn off soot – stop-start urban driving increases failure risk.

Hybrid Engine MOT Failures

Hybrids combine electric power with a petrol or diesel engine, so their MOT failures differ again:

✔ Electrical system faults:

Battery issues, powertrain error codes, and charging faults.

✔ Brake wear:

Hybrids use regenerative braking, which can hide brake issues until MOT time.

✔ Engine inactivity issues:

If the engine rarely runs, fuel system components may degrade unnoticed.

Hybrids often pass emissions smoothly but fail due to complex electrical or software-related systems.

Check Vehicle MOT History for Free

You can instantly check MOT history and status by visiting CheckMot.com – a useful step before purchase or booking your next test.

FAQs

Do hybrids need an MOT emissions test?

Yes, but they usually emit lower levels, so they pass emissions more easily than petrol or diesel vehicles.

Why do diesel cars fail more than petrol?

Diesels face stricter emissions standards and have more complex components like DPFs that can easily fail.

Can I drive after an MOT fail?

Only if your previous MOT is still valid AND the car is roadworthy. Otherwise, it’s illegal.

Is a DPF removal legal?

No. Removing or tampering with a DPF results in an automatic MOT fail and fines.