P0455 — Fault Tree

P0455 — Evaporative emission system leak detected (large)

The fuel vapour system is not holding pressure. The single most common cause is a gas cap that was not tightened, which is why this one is worth checking before it is worth paying for.

System: Powertrain
Definition: Generic (SAE-defined, all manufacturers)
Subsystem: Auxiliary emission controls
Severity: minor

Causes, by prior probability

Cause	Prior	Repair
Fuel cap loose, cracked or wrong Tighten until it clicks, clear the code, drive two drive cycles. Free to test.	42%	$36
Cracked EVAP hose Smoke test the EVAP system with the purge valve commanded closed.	24%	$162
Purge valve stuck open Command the valve closed and watch for vacuum decay.	19%	$173
Vent valve or canister failure Bidirectional control test on the vent solenoid.	15%	$350

These are priors from the code alone. A scan's freeze frame reorders them, which is the entire point of capturing it.