fault code guide
P0456: EVAP System Very Small Leak Detected
Your car has detected a tiny escape of petrol fumes from the emissions vapour system, often around the fuel cap or a hose. You can safely keep driving for now, but you should have it repaired before your next test.
How it shows up: A steady amber engine management light illuminates on the dashboard, which requires an OBD scanner to identify as code P0456. The car usually drives completely normally, though you may occasionally notice a faint petrol vapour odour near the fuel flap.
Updated 9 November 2026
Can I keep driving?
Service soon
Your car remains safe to drive without immediate risk of breakdown, but fuel vapours are venting into the air and the light will prevent your car from passing an MOT.
What to do right now
- 1Park safely, switch off the ignition, and check that your fuel cap is fastened tightly until it clicks.
- 2Inspect the rubber seal on the fuel cap for obvious splits, hardening, or grit.
- 3Continue driving as normal if the engine runs smoothly and the amber engine light remains steady rather than flashing.
Likely causes
Most likely first.
- Likely
Worn or loose fuel filler cap
The rubber o-ring on the fuel cap hardens, cracks, or collects road grime over time. This prevents a tight seal, allowing a tiny amount of petrol vapour to bleed out during self-tests.
- Possible
Perished or cracked vapour line
Rubber and plastic vapour hoses run between the fuel tank, charcoal canister, and engine. Heat and age cause hairline micro-cracks that allow pressure to drop very slowly.
- Possible
Leaking EVAP purge control valve
This valve meters fuel vapours into the engine intake manifold. If carbon build-up or internal spring fatigue stops it from sealing completely shut, the system registers a minor pressure leak.
- Rare
Corroded fuel filler neck
Road salt and trapped moisture can corrode the metal lip of the filler neck where the cap seats. The surface becomes uneven, preventing the fuel cap seal from creating an airtight fit.
What you can check yourself
Park somewhere safe and level, engine off, before you start.
- 1
Inspect the fuel cap rubber seal
Unscrew your fuel cap and look closely at the circular rubber gasket. If it feels rock-hard, shows fine cracks, or is misshapen, replacing the cap will usually cure the fault.
- 2
Clean the filler neck mating face
Use a clean, lint-free cloth to wipe away any grit, dust, or light rust from the rim inside the fuel flap where the cap presses against metal. Even tiny grains of sand can cause a micro-leak.
- 3
Check under-bonnet vapour hoses
With the engine cold and switched off, pop the bonnet and locate the thin plastic or rubber hoses running near the intake manifold. Check for loose push-fit clips, chafing, or split rubber ends.
- 4
Allow for three complete drive cycles
After reseating the cap, drive the car over several days with trips that include both urban driving and motorway cruising. The engine control unit only tests this system under specific cruising and fuel level conditions.
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 diagnostic labour, as locating a microscopic leak often requires a specialist smoke machine test.
Replacement fuel filler cap
- Do it yourself
- €15–45£12–40 in the UK
- At a garage
- €50–95£45–85 in the UK
EVAP purge control valve replacement
- Do it yourself
- €40–110£35–95 in the UK
- At a garage
- €130–240£110–210 in the UK
EVAP smoke test diagnosis and hose repair
- Do it yourself
- Garage job
- At a garage
- €140–290£120–250 in the UK
When to go to a garage
- You have replaced the fuel cap and the fault code returns after a week of normal driving.
- You smell raw petrol vapour inside the cabin while driving or when standing beside the vehicle.
- Your car is due for its annual MOT or NCT within the coming weeks.
Questions drivers ask
Can I continue to drive my car with a P0456 code?
Yes, you can safely drive your car with this code active. A very small evaporative leak does not affect engine performance, braking, or safety systems, and it will not damage your catalytic converter. However, it should be resolved before long because it allows unburnt hydrocarbons to pollute the environment.
Will code P0456 cause my car to fail the MOT or NCT?
In the UK, a lit engine management light on a petrol car first registered from July 2003 is an automatic MOT fail. In Ireland, an illuminated engine light on its own results in an Advisory Pass on the NCT, failing only if exhaust tailpipe emissions exceed legal thresholds or the tester cannot complete the evaluation.
How does code P0456 differ from P0442 and P0455?
All three codes relate to evaporative emissions leaks, but they differ by the physical size of the opening detected. P0455 denotes a gross leak such as a missing cap, P0442 indicates a small leak, and P0456 signifies a very small micro-leak typically under half a millimetre wide.
Why does the warning light for P0456 disappear and return intermittently?
The engine computer only runs the micro-leak test under very specific operating conditions, typically when the fuel tank is between one-quarter and three-quarters full during steady cruising. If ambient temperatures shift or your fuel level falls outside that bracket, the test aborts, causing the light to stay off until the parameters are met again.
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.