fault code guide
P0455: EVAP System Emission Control System Leak Detected (Gross Leak)
Your car has detected a substantial fuel vapour leak in the evaporative emissions system. The car remains safe to drive normally, but you should inspect the fuel cap and arrange a check.
How it shows up: An amber engine management light will stay illuminated on your dashboard, and you may notice a faint petrol smell near the fuel flap. The specific P0455 code is confirmed by connecting an OBD diagnostic scanner.
Updated 9 November 2026
Can I keep driving?
Service soon
The car will run normally, but leaking petrol vapours harm the environment, waste fuel, and will lead to an MOT test failure.
What to do right now
- 1Pull into a safe location and ensure the fuel cap is securely tightened until it clicks.
- 2Walk around the vehicle to check whether you can smell strong raw petrol or spot any liquid drips underneath.
- 3If you see puddling fuel or smell heavy petrol fumes, turn off the ignition and call roadside assistance.
- 4If there is no physical leak and the car drives normally, continue your trip and arrange an inspection.
Likely causes
Most likely first.
- Likely
Loose, damaged, or missing fuel cap
A cracked rubber seal, cross-threaded cap, or loose cap prevents the fuel tank from holding a vacuum during automated system checks.
- Possible
Faulty EVAP purge or vent valve
If an electric purge valve or vent solenoid sticks wide open, air flows unchecked through the system and the engine computer registers a gross leak.
- Possible
Rusted fuel filler neck or cracked vapour hose
Corrosion on steel filler pipes or split rubber lines near the fuel tank allow atmospheric air to enter freely.
- Rare
Damaged charcoal canister
Road debris or stone chips can crack the plastic charcoal canister housing mounted beneath the vehicle floor.
What you can check yourself
Park somewhere safe and level, engine off, before you start.
- 1
Inspect the fuel cap and seal
Unscrew your fuel cap and check the black rubber sealing ring for splits, flattening, or grit. Refit it firmly until you hear several distinct clicks.
- 2
Examine the fuel filler neck
Look inside the fuel flap with a torch for visible rust, flaking metal, or physical damage around the neck opening where the cap mates.
- 3
Check under the bonnet for loose pipes
With the engine off and cool, inspect the thin plastic and rubber hoses near the intake manifold. Look for any disconnected or severely cracked vacuum lines leading to the EVAP purge valve.
- 4
Check underneath for wet spots
Look under the rear of the car near the fuel tank for damp patches or wet fuel lines, taking care never to use an open flame nearby.
Rough cost to fix
Rough estimateTypical prices in Ireland (€) and the UK (£), from an independent garage up to a main dealer. Prices vary depending on whether the fault is a simple cap replacement or a rusted filler pipe requiring fuel tank removal labour.
Replacement fuel filler cap
- Do it yourself
- €15–45£12–40 in the UK
- At a garage
- €35–80£30–70 in the UK
EVAP purge control valve replacement
- Do it yourself
- €40–120£35–105 in the UK
- At a garage
- €120–240£100–210 in the UK
Fuel filler neck replacement
- Do it yourself
- Garage job
- At a garage
- €250–500£210–430 in the UK
Evaporative charcoal canister replacement
- Do it yourself
- Garage job
- At a garage
- €220–480£190–410 in the UK
When to go to a garage
- You smell intense petrol fumes inside the passenger cabin while driving.
- You notice liquid fuel dripping or pooling beneath the rear of the car.
- You have replaced the fuel cap and the warning light returns after clearing it with a scanner.
- A garage smoke-machine test is required to trace a leak hidden above the fuel tank.
Questions drivers ask
Can I keep driving my car with code P0455?
Yes, you can usually continue driving because this code does not stop the engine running or risk mechanical breakdown. However, if you smell strong petrol vapour or spot liquid fuel dripping, stop immediately and seek assistance due to fire risk. Otherwise, book an inspection within a couple of weeks.
Will my car fail the NCT or MOT with a P0455 code?
On the UK MOT, a lit engine management light is an automatic failure on petrol cars first used from July 2003. In Ireland, the NCT considers an illuminated engine light or stored emissions code an Advisory Pass, meaning the car only fails if exhaust emissions exceed legal limits or if fuel vapour issues prevent testing.
How does code P0455 differ from P0442 and P0456?
All three codes indicate evaporative emissions leaks, but they specify different leak dimensions. Code P0455 denotes a gross leak equivalent to an opening larger than 2.3 mm, whereas P0442 represents a small leak and P0456 is a very small pinhole leak. A completely detached hose or loose cap usually triggers P0455.
Does tightening the fuel cap make the engine light go out immediately?
No, tightening the cap rarely turns the warning light off straight away. The engine control unit must run and pass several automated vacuum tests over multiple driving cycles before it clears the light. You can clear the fault code instantly using an OBD diagnostic tool once the cap is secure.
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.