Codes and warning lights
Why does the check engine light come on after refueling?
The most common cause is a gas cap that is not closed properly or has a damaged seal: on cars that monitor the tank's vapors, the control unit detects a leak and stores a code such as P0455 or P0457. Less often it is poor-quality or wrong fuel, which causes misfires or mixture faults.
Reviewed by Daniel Morales, founder of Reprorace SL, on
Why does a loose cap turn on the light?
The evaporative system (EVAP) collects gasoline vapors from the tank in a charcoal canister and burns them in the engine. To check for leaks, the control unit seals it and measures pressure or vacuum.
If the cap is loose, missing or has a cracked seal, the test fails. Not every European car runs this test, but on those that do it is the most common reason for the light after refueling.
What should I do if it came on?
When it is safe to stop, check that the cap is on and tight (until it clicks, if it is that type) and that the seal is not damaged. On capless fillers, check the neck is free of dirt.
The light does not go off right away: the control unit has to repeat the test over several drive cycles. If it is still on after a few days, read the code.
What if the car runs badly after refueling?
If it stumbles, shakes or loses power, the fuel may be contaminated or poor quality. If you put in the wrong fuel and notice before starting, do not start the engine and call for assistance.
A flashing light means severe misfires: slow down and go to a workshop.
General guidance only: every manufacturer and model has its own details. Always follow your vehicle's manual and, if in doubt, ask a workshop.
Related
Related trouble codes
- P0457EVAP system: leak detected (fuel cap loose or missing)
- P0455EVAP system: large leak detected
- P0456EVAP system: very small leak detected
- P0442EVAP system: small leak detected
- P0441EVAP system incorrect purge flow: malfunction
- P0300Random or multiple cylinder misfire detected
Related symptoms
- Engine warning light on and steady: what it means and what to do
- Smell of gasoline or diesel in the car: what to do
Learn more
See what your car is saying
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Frequently asked questions
How long until the light goes off after tightening the cap?
It depends on the car: it has to repeat the test, which can take several trips or days.
Can I just clear the code?
You can, if you are sure it was the cap, but the monitors will be incomplete for a few days. If it comes back, there is a real leak.
Is it dangerous to drive like this?
A vapor leak does not usually affect driving. If you smell strong fuel, stop and have it checked.