The fuel cap: codes P0455, P0442 and P0457
Three codes whose culprit costs nothing in a third of cases and is fixed by hand in five seconds.
Petrol always evaporates inside the tank. To keep the vapour out of the air a charcoal canister collects it, and the system watches for leaks: once per trip it checks whether the tank holds vacuum.
If it does not, one of three codes arrives:
- P0457 — plainly and honestly: the cap is loose or missing;
- P0455 — a large leak: the cap does not seal at all, a hose is torn, the canister is cracked;
- P0442 — a small leak: most often a hardened seal on the cap.
What to do yourself
- Undo the cap and tighten it until it clicks. On many cars leaving it one click loose at the pump is enough.
- Look at the rubber ring. A cracked or hardened seal is the most common cause of P0442. A cap costs very little.
- Drive for a few days. The leak test does not run every minute but under conditions — a certain fuel level and temperature. The code clears itself, but not immediately.
When the cap is not to blame
If the cap is new and the code returns, the next suspects need a lift: chafed hoses along the floor, the canister purge valve and the vent valve — P0446, P0447 and P0448 are precisely about those. The vent valve lives underneath and seizes up from road salt.
These codes do not affect driving; nothing is going to break. But there is only one warning lamp for every occasion: while it is busy with your fuel cap, you will not notice something serious appearing beside it. That alone is a reason to clear them.
One parting habit: after refuelling, tighten the cap until it clicks. This is the only code in the book that is cured by a habit.
