What Is P0455?
P0455 means the ECU has detected a large leak in the Evaporative Emission Control (EVAP) system. The leak threshold for P0455 is equivalent to approximately 0.080 inches (2mm) or larger — double the P0442 threshold. A large leak means the EVAP system fails to hold any meaningful pressure during the ECU self-test.
The most common cause of P0455 is a missing, loose, or damaged fuel filler cap. The cap forms part of the sealed EVAP system, and a missing or poorly-sealing cap creates an open vent to atmosphere that the ECU detects immediately. Always check the fuel cap first before any other diagnosis — it resolves P0455 in a large proportion of cases.
Common Symptoms
- Engine management light on
- Possible strong fuel vapour smell
- No driveability impact
- No performance loss
Common Causes
How to Diagnose P0455
Check Fuel Cap
Remove and firmly refit the fuel cap — click it until it stops turning. Inspect the cap seal ring for deterioration or damage. If the seal is cracked or missing, replace the cap (£10–£20). Clear the code and retest.
Inspect EVAP Hoses for Disconnection or Major Splits
Check all visible EVAP system hoses for complete disconnection or large splits. P0455 is typically obvious — look for a hose that has completely come off a fitting.
Check Charcoal Canister
Inspect the canister body for physical damage, particularly if the vehicle has been grounded recently. A cracked canister is visible on inspection.
Test Vent Valve Operation
Apply vacuum to the vent valve with it de-energised (closed). It should hold vacuum. If it bleeds down immediately to zero, the valve is stuck open and needs replacement.
Smoke Test
If no obvious large leak is found, a smoke test will quickly locate the leak point in the EVAP system.
Mechanic's Corner — P0455 on UK Cars
P0455 (large EVAP leak) is a more serious indication than P0442. A leak large enough to trip P0455 is usually visible or audible — a missing fuel cap, a completely disconnected EVAP hose, or a cracked charcoal canister are the typical causes. On UK cars that have been recently serviced or worked on, always check that the fuel cap was refitted correctly and that any disturbed EVAP hoses were reconnected. A forgotten fuel cap or knocked-off hose is the most embarrassing but also the most common cause of P0455.
On higher-mileage vehicles, particularly Japanese imports that ran on a different fuel spec, the charcoal canister can become saturated and allow liquid fuel to enter the EVAP system, which creates both P0455 and a persistent fuel vapour smell in the cabin. A saturated canister must be replaced — cleaning is not effective. The canister is usually located near the fuel tank and is a relatively inexpensive part (£40–£100).
Verdict
Check the fuel cap first — it is the cause of P0455 more often than any other component. A new fuel cap is £10–£20. If the cap is confirmed good, look for a completely disconnected or split hose next. P0455 should be an obvious, large leak that is findable by inspection.
