EVAP Large Leak
P0455 means the EVAP system failed a leak check badly enough that the computer sees a large leak or no pressure/vacuum change.
It is usually not a drivability emergency, but fuel vapor leaks should not be ignored.
Common symptoms
- Check engine light
- Fuel smell
- Usually no performance issue
- Failed emissions readiness
Likely causes
- Loose/missing gas cap
- Broken EVAP hose
- Stuck-open purge valve
- Bad vent valve
- Cracked charcoal canister
What to check before replacing parts
Check the gas cap first
Look for: A loose, missing, or cross-threaded cap is by far the most common cause of a large EVAP leak code — if it doesn't click to full tightness or the seal is cracked, that's very likely the whole fault.
Inspect visible EVAP hoses
Look for: A large leak is often obvious: a hose disconnected at a fitting, split open, or chewed through (rodent damage is common) rather than a hairline crack.
Command the purge and vent valves with a scan tool
Look for: A valve that doesn't respond at all when commanded open/closed, or stays open when it should seal, prevents the system from holding pressure/vacuum and reads as a large leak.
Smoke-test the system
Look for: With a large leak, expect an obvious plume of smoke or a clearly audible hiss at the fault location — it should be easy to spot compared to a small-leak diagnosis.
Inspect the canister and filler neck
Look for: A cracked charcoal canister housing or a corroded/rusted-through filler neck (common on older vehicles) can leak enough to prevent the system from ever holding vacuum.
This page explains the generic P0455 definition and is general reference information, not a diagnosis for your specific vehicle. Actual causes, severity, and repairs vary by make, model, engine, and mileage — always confirm with a scan tool and a qualified technician before replacing parts.
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Matching symptoms
Related OBD2 codes
P0455 on your vehicle
Causes ranked from real NHTSA owner complaints for these specific vehicles: