Summary
P0455 means the PCM detected a large leak in the EVAP system, which captures fuel vapor from the gas tank to prevent it from escaping into the atmosphere. The most common cause is a loose or missing gas cap (50% of cases), followed by a cracked or disconnected EVAP hose (25%) and a faulty purge or vent valve (15%). Start by tightening or replacing the gas cap ($5–$15) and clearing the code.
Severity: Low — not a drivability or safety issue
Safe to drive: Yes — this is an emissions code only
Repair cost: $5–$400 depending on cause
DIY difficulty: Easy to Moderate
What does P0455 mean?
The EVAP (Evaporative Emission Control) system is a sealed network of hoses, valves, and a charcoal canister that captures fuel vapors from your gas tank instead of letting them escape into the air. The PCM periodically tests this system for leaks by pressurizing or pulling a vacuum on the system and monitoring whether it holds.
P0455 specifically indicates a "large" leak — the system lost pressure faster than the threshold for a small leak (that would be P0442). In practice, "large" often means something as simple as a gas cap that's loose, cracked, or missing entirely.
This code does not affect engine performance. Your engine will run exactly the same with or without an EVAP leak. The system exists solely for emissions control.