EVAP leaks and the fuel cap
Petrol cars catch the fuel vapour from the tank and burn it in the engine. Many test that system for leaks themselves, and a fuel cap that doesn’t seal is the first thing to check.
Updated
What the EVAP system does
Petrol gives off vapour. Instead of venting it into the air, the evaporative emission (EVAP) system leads it from the tank into a charcoal canister, which holds it. With the engine running, a purge valve lets the engine draw the vapour in and burn it, and a vent valve lets fresh air into the canister.
How the leak test works
To check for leaks, the engine computer closes the system off and watches the pressure in the tank. Depending on the car, it draws a vacuum through the purge valve, uses a small pump, or waits after you switch off and watches how the pressure changes as the tank cools. If the pressure or vacuum doesn’t hold, there’s a leak.
- P0455
- Large leak, such as a missing or loose fuel cap.
- P0442
- Small leak.
- P0456
- Very small leak.
- P0457
- Leak found, and the engine computer points to the fuel cap.
- P0440
- A fault in the EVAP system that doesn’t fit a more specific code.
Why it takes days
The leak test only runs under particular conditions, which differ between makes. Typical ones are a fuel level that’s neither nearly empty nor nearly full, temperatures within a range, a start after the car has stood for hours, and a drive of some length. It usually runs at most once per drive, and some cars need several tests before they turn the light on or off.
So a leak can take days to show up, and after a repair the light can take days to go off. The evaporative system self-test is often one of the last to complete after the codes are cleared.
Many European cars don’t run it
The leak test comes from the US rules. European OBD rules only require the purge valve’s circuit to be checked, not the system for leaks. Many European petrol cars therefore have no EVAP leak test at all, although some do.
If your car doesn’t run the test, Read My Car usually counts the evaporative system self-test among the ones that don’t apply to this engine.
Check the fuel cap first
- Take the cap off and look at the seal: no cracks, tears or flattened spots, and nothing stuck to it.
- Look at the filler neck where the seal sits: clean and undamaged.
- Close the cap properly, until it clicks if it’s that kind. Replace it with the right type for your car if the seal is worn.
- Drive normally for a few days, then read the codes again. A pending code that keeps coming back means the leak is still there.
On cars with a capless filler, check the filler flap for dirt instead. Clearing the code straight away resets the self-tests too; on many cars the light goes off by itself once the test has passed again.
If it isn’t the cap
Small leaks are usually too small to see or hear. A garage then does a smoke test: it fills the system with harmless smoke at low pressure and watches where it comes out. They also check the purge and vent valves, the canister and the hoses around the tank.
Questions
Can I drive with an EVAP leak code?
Usually, for now: the car drives as before, but fuel vapour escapes and an inspection that reads OBD sees the code. A strong smell of fuel or fuel dripping is different: don’t drive, and have it checked.
I tightened the fuel cap. When does the light go off?
Often only after the leak test has run and passed again, which can take several days of normal driving. If the code comes back, the leak is elsewhere.
Does my car run an EVAP leak test?
Look under Vehicle, emissions readiness in Read My Car. If the evaporative system isn’t among the self-tests, your car doesn’t run it; many European cars don’t.