Code P0455 / P0442: evaporative system leak, check the fuel cap first

Fault codes · Updated on · 3 min read · Saïd, creator of DiagAuto AI

What the code means

On a petrol car, fuel vapours are stored in a charcoal canister (activated carbon filter) and then burnt by the engine. To check that no vapour escapes, the control unit puts the system under a slight vacuum and watches whether it holds. If it drops quickly, that is a large leak (P0455); if it drops slowly, a small leak (P0442, P0456).

Is it serious?

Not for the engine. You lose a little fuel through evaporation and may sometimes smell petrol. The only real problem: warning light on = re-test at the roadworthiness test (contrôle technique in France, similar to the UK MOT).

The most common causes

  1. Fuel cap: not tightened, cracked seal, non-original cap.
  2. EVAP hoses split or disconnected (under the car, near the tank, in the engine bay).
  3. Purge valve (engine side) or vent valve (canister side) stuck.
  4. Canister cracked or saturated after overfilling the tank.
  5. Rusty filler neck (older cars).

What to check first

  1. Remove and refit the fuel cap, tightening until it clicks. Look at the seal: if it is hard or cracked, replace the cap (€15 – 40).
  2. Clear the code with DiagAuto AI (after confirmation; the app does no reprogramming) and drive several trips (the leak test only runs under certain conditions: tank between 1/4 and 3/4 full, cold engine at start-up).
  3. If the code comes back, inspect the visible hoses. A fault can also be heard: with the ignition on, listen for the purge valve clicking.
  4. At the garage, a smoke test locates the leak in a few minutes.

Should you clear the code?

Yes, that is actually the diagnostic method: clear, drive, see if it comes back.

Indicative cost (France, 2026)

Fuel cap: €15 – 40. Hose: €20 – 80. Purge valve: €40 – 150 fitted. Canister: €150 – 400 fitted. Smoke test: €40 – 80.

Read also: Clearing a fault code: when it helps and 7 repairs you can do yourself after a diagnosis.

Frequently asked questions

P0455 appeared right after filling up. Is that normal?

Very common: the cap was not closed properly, or the tank was topped up to the brim and flooded the canister. Tighten the cap until it clicks and drive a few trips; the code may go away on its own.

Does this code apply to diesels?

Almost never: the EVAP system collects petrol vapours. On a diesel, a code in the P044x family means something else; look it up in the code dictionary.

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