fault code guide
P0442: Evaporative Emission Control System Leak Detected (Small Leak)
The engine computer has detected a tiny escape of petrol fumes from the fuel vapour recovery system. The car will drive normally, but you should resolve the leak to prevent fuel smell and pass emissions testing.
How it shows up: The amber engine management light stays on steadily on your dashboard, and the car usually runs without any noticeable loss of power. A mechanic or diagnostic tool reads the fault code P0442 from the OBD port.
Updated 9 November 2026
Can I keep driving?
Service soon
Your car remains safe to drive without immediate engine risk, but unburnt petrol vapours escape into the air and will cause an MOT failure.
What to do right now
- 1Check your instrument cluster to confirm the engine management light is glowing steady amber rather than flashing.
- 2Pull into a safe parking spot, turn off the engine, and check that your fuel cap is fully tightened until it clicks.
- 3Inspect the ground around your fuel tank and engine bay to ensure there is no liquid fuel dripping or strong petrol stench.
Likely causes
Most likely first.
- Likely
Loose, worn or missing fuel cap
A cracked rubber seal or an improperly clicked fuel filler cap prevents the fuel tank from holding a vacuum during automated system self-tests.
- Possible
Stuck or leaking EVAP purge valve
The purge valve in the engine bay can stick slightly open due to carbon dirt, letting vacuum bleed off when it should remain sealed.
- Possible
Perished or cracked vapour line hose
Rubber and plastic vapour lines run the length of the car chassis, where road grime, age, and heat can cause tiny pinhole cracks.
- Rare
Damaged charcoal canister or vent valve
Road debris can strike the charcoal canister mounted beneath the car near the fuel tank, causing fine hairline fractures in the plastic body.
What you can check yourself
Park somewhere safe and level, engine off, before you start.
- 1
Inspect the fuel cap rubber seal
Park on level ground with the engine switched off. Unscrew the fuel cap and inspect the rubber seal for splits, hardening, dirt or cracks. Clean the rubber ring with a dry rag, refit it firmly, and turn it until you hear three distinct clicks.
- 2
Check the fuel filler neck rim
Look closely at the metal rim where the fuel cap seals against the filler pipe. Wipe away any accumulated road grit, rust flakes or dirt that could stop the rubber gasket from sealing flat.
- 3
Look over visible engine bay vapour lines
With the engine cold and switched off, open the bonnet and locate the thin plastic and rubber hoses near the intake manifold. Look for any loose connectors, split rubber elbows or brittle pipes that have slipped off.
- 4
Sniff around the rear wheel arch
Walk around the car near the fuel flap after parking. A distinct petrol smell near the rear wheel arch points towards a cracked pipe or a failing charcoal canister.
Rough cost to fix
Rough estimateTypical prices in Ireland (€) and the UK (£), from an independent garage up to a main dealer. The total cost depends mainly on how quickly a garage can locate the pinhole leak using a dedicated smoke machine.
Replacement fuel filler cap
- Do it yourself
- €15–45£12–40 in the UK
- At a garage
- €40–90£35–80 in the UK
EVAP purge control valve replacement
- Do it yourself
- €35–110£30–95 in the UK
- At a garage
- €110–230£95–200 in the UK
EVAP smoke test and split vapour hose repair
- Do it yourself
- Garage job
- At a garage
- €120–260£100–220 in the UK
Charcoal canister replacement
- Do it yourself
- Garage job
- At a garage
- €220–450£190–390 in the UK
When to go to a garage
- Book an appointment if tightening or replacing the fuel cap does not turn off the engine light after several days of driving.
- Head to a garage straight away if you smell strong petrol vapours inside the vehicle cabin.
- Call for roadside assistance if the engine light starts flashing or the engine begins to misfire and shudder.
Questions drivers ask
Can I drive my car with fault code P0442?
Yes, you can safely continue your trip because a small vapour leak does not affect engine lubrication or mechanical reliability. However, you should arrange an inspection within a few weeks to avoid failing emissions tests and wasting petrol vapours into the air.
Will clearing the code turn the engine light off permanently?
Clearing the code with an OBD scanner only removes the light temporarily. The car tests the fuel vapour system under specific driving conditions, usually when the tank is between one-quarter and three-quarters full, and the warning light will return once the leak is redetected.
What is the difference between fault code P0442 and P0455?
Code P0442 indicates a small leak, typically defined as an opening around one millimetre across, such as a dried seal or hairline crack. Code P0455 signals a gross or large leak, which usually happens when the fuel cap is left off completely or a vapour hose has disconnected entirely.
Will fault code P0442 cause my car to fail the NCT or MOT?
On the UK MOT, a lit engine management light on post-2003 petrol cars results in an automatic test failure. On the Irish NCT, engine fault codes alone result in an advisory pass unless the issue causes exhaust emissions to exceed legal limits or prevents testing.
Test rules here are for the National Car Test in Ireland (NCT) and the MOT in the UK. Elsewhere, check the regulations of your own country or state before going for a vehicle test.
The warning light you may see
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General guidance for most cars, not a diagnosis of yours. Details differ by make, model and year - check your handbook, and if in doubt, don't drive.