P0456: Evaporative Emission System Leak Detected (Very Small Leak)
The evaporative emission system has a very small leak.
Normal risk
Safe to drive. Fix it before an inspection.
Short answer · Can I keep driving with P0456?
Yes. It is not a safety risk. The fuel cap seal is the first thing to check; very small leaks often need a smoke test to find.
What does P0456 mean?
Code P0456 means the system test found a very small leak (often about 0.5 mm or 0.020 inch, depending on the manufacturer). These are hard to find without a smoke machine.
What the car does when the code is set
The check engine light comes on. Driving is not affected. Some fuel vapour escapes into the air, and in many countries the car fails the emissions part of an inspection while the code is active.
How the system works
Petrol gives off vapour, especially when warm. Instead of letting it escape, the evaporative emission system stores it in a charcoal canister. When the engine runs under suitable conditions, the purge valve opens and the engine draws the vapour in and burns it. To make sure the system is sealed, the engine control unit runs a leak test, usually after the car has been parked: it closes the vent valve and either creates a vacuum or applies pressure, then watches whether it holds. The size of the leak it detects decides which code is stored.
Common symptoms
Possible causes
Listed without ranking. The cause on your car depends on make, engine and mileage — see the maker notes below.
- Loose, damaged or wrong fuel capThe most common and cheapest cause.
- Cracked hose or loose connectionHoses between the tank, canister and engine age and crack.
- Faulty purge valveA valve that does not seal lets vapour escape.
- Faulty vent valveThe vent valve on the canister does not close during the test.
- Leaking filler neck or canisterRust on the filler neck or a cracked canister.
How it is diagnosed
- Check the fuel capCheck the seal, close the cap until it clicks, and clear the code. The test runs again over several drive cycles.
- Inspect visible hosesLook at the hoses near the canister and engine for cracks and loose connections.
- Smoke-test the systemA workshop fills the system with smoke at low pressure; the smoke shows where it escapes.
- Test the valvesPurge and vent valves are tested with a scan tool or by applying voltage, as described in the service data.
Questions about P0456
Is an evaporative leak dangerous?
Small leaks are not a direct danger, but they release fuel vapour. A strong fuel smell, especially inside the car, should be checked straight away.
Will the car fail an inspection?
In many countries a lit check engine light fails the emissions test, even for a small evaporative leak.