P0455 — EVAP System Large Leak Detected (MG Context)
Standard meaning: the evaporative-emissions system — which stores fuel-tank vapours and burns them in the engine instead of venting them — failed its leak self-test badly. Before anything else: remove and refit the fuel cap until it clicks, and drive a few days. A loose or ageing cap is the single most common cause of this code across every brand, and it costs nothing to rule out.
The usual suspects — cheapest first
- Fuel cap — loose after a refuel, or its seal aged and cracked. Also: caps left dangling scratch their sealing face over years.
- Refuelling with the engine running — triggers the self-test at exactly the wrong moment on some cars. Don’t.
- Cracked EVAP hoses — the same rubber-age story as everything else on an older car.
- Purge or vent valve stuck — the actual component failure, diagnosed properly with the cheap causes eliminated.
MG-specific context
No documented MG-specific EVAP pattern exists in our research to date, and we say so plainly rather than manufacture one. One indirect note for older MG3 owners (2021 build): the fuel-tank strap recall (REC-001632 / R/2021/216) involved work around the tank on affected cars — if EVAP codes appeared right after recall work, mention that timing to the dealer; disturbed hose connections are a mundane but real possibility after any tank-area job.
Safe owner checks
- Cap off, inspect the seal for cracks, refit to the click. Clear expectations: the self-test may take several drive cycles to pass, so the light won’t vanish instantly.
- Fuel smell near the filler or in the boot? That upgrades this from “whenever convenient” to “this week”.
- Was the tank recently brimmed to the top repeatedly? Overfilling can saturate the charcoal canister — a habit worth breaking.
When it’s a workshop job
EVAP smoke-testing finds the leak in minutes with the right machine. Valve and canister replacement are routine. There’s no safety drama here — just don’t ignore a fuel smell.