P0455 Code: Evaporative Emission System Leak Detected (Large Leak)
Last reviewed May 2026 · Reviewed by the Find This Code Editorial Team
Quick Answer
P0455 means the EVAP system detected a large leak in the fuel vapor containment system. Despite the word 'large,' this is often caused by something as simple as a loose or improperly seated gas cap. The car will run normally, but it will fail an emissions test. Compare: P0442 is a small leak, P0455 is a large leak, P0456 is a very small leak.
At a Glance
What this code means
The EVAP system captures fuel vapors from the tank and fuel system, routes them through a charcoal canister, and purges them into the engine's intake to be burned — rather than releasing them as pollution. The ECM periodically tests the integrity of this sealed system by monitoring for pressure decay. P0455 means the ECM detected a leak larger than approximately 0.040 inches in diameter. Despite the 'large leak' label, this threshold is still quite small — a loose gas cap is often enough to trigger P0455. However, larger physical failures like a disconnected EVAP hose, a stuck-open purge valve, or a cracked canister will also set this code. The key difference between P0455 and P0442 is the detected leak size: P0455 loses pressure faster, pointing to a larger opening, while P0442 indicates a smaller, harder-to-find leak.
The EVAP system is sealed. The ECU pressurizes it and checks for leaks. Any escape point — gas cap, hose, valve — triggers an EVAP code.
Common causes
- Loose, improperly seated, or damaged gas cap — the single most common cause
- Disconnected or cracked EVAP hose or vapor line
- Failed EVAP purge valve stuck open — prevents the system from sealing during the EVAP monitor test
- Failed vent valve stuck open or not sealing
- Damaged, cracked, or saturated charcoal canister
- Damaged fuel filler neck or missing fuel cap tether
- Cracked fuel tank (less common but produces a large leak)
Symptoms you might notice
- Check engine light — typically the only symptom in most cases
- Possible fuel smell near the rear of the vehicle or around the fuel cap
- Failed emissions test — P0455 will fail an OBD-II emissions inspection
- No rough running, power loss, or drivability problems in the vast majority of cases
Can you still drive?
Yes. P0455 is a pure emissions fault with no immediate safety or driveability risk. The engine will run normally. Address it before an emissions inspection or if you notice a persistent fuel smell.
How serious is this code?
Low. No mechanical or safety risk. The code will not clear on its own — the EVAP monitor must pass before the light turns off. Plan to diagnose and fix the leak.
Before you replace parts
Parts replacement is often not the first step. Before buying anything, it's worth checking these basics:
- Re-seat and tighten the gas cap, clear the code, and drive for 2–3 days — this resolves a very large share of P0455 cases without any other repair
- Inspect the cap's rubber O-ring seal — a flat, cracked, or dirty seal won't hold pressure even when tightened. A $10–$25 cap is the cheapest possible fix
- If the code returned shortly after refueling with a full fill-up, you may have overfilled the tank past the auto-shutoff — this forces liquid fuel into the charcoal canister and can trigger EVAP codes temporarily
- Confirm the purge valve seals properly before replacing the charcoal canister — a stuck-open purge valve is far cheaper to replace ($50–$120) and is a more common failure than the canister itself
How to troubleshoot it
- Start with the gas cap: remove it, inspect the rubber O-ring for cracks or compression, and reinstall firmly. On ratcheting caps, turn until you hear 3+ clicks
- Clear the code after addressing the cap and drive normally for 2–3 days — if the cap was loose, the EVAP monitor will run and pass without further action
- Inspect all accessible EVAP hoses for cracks, kinks, or disconnections — particularly the large vapor line running from the fuel tank to the charcoal canister
- Check the purge valve: a purge solenoid stuck in the open position allows air into the system continuously and prevents the EVAP monitor from completing
- Check the vent valve on the charcoal canister — it should be normally open when not energized; if it's stuck closed or stuck open at the wrong time, the monitor test fails
- If the above are clear, have a shop perform a smoke test — they pressurize the EVAP system with smoke and locate the leak visually
Common mistakes to avoid
These are the most frequent diagnostic errors when dealing with P0455. Avoiding them can save time and money.
- Replacing major EVAP components (charcoal canister, purge valve) before verifying the gas cap — a loose or cracked gas cap causes the majority of P0455 codes
- Using a cheap aftermarket gas cap that doesn't seal to the correct specification and re-triggers the code
- Ignoring the code because the car drives fine — EVAP leaks cause emissions test failures and tend to worsen over time
- Trying to locate the leak visually instead of using a smoke machine — EVAP system leaks are rarely visible without pressurization
Tools that may help
These are the types of tools commonly used when diagnosing this code. Having the right tool can save time and help confirm a diagnosis before spending money on parts.
- OBD-II scanner
- EVAP smoke machine (shop tool)
- Gas cap pressure tester (optional)
Estimated repair cost range
Gas cap: $10–$30. EVAP hose repair: $50–$150. Purge or vent valve replacement: $100–$250. Smoke test at shop: $75–$150. Charcoal canister: $150–$400.
Costs vary significantly by vehicle, location, and whether you use OEM or aftermarket parts. These are general ranges for reference only.
When to call a mechanic
If you've completed the basic troubleshooting steps and the code keeps returning, or if you don't have access to the proper diagnostic tools, it's worth consulting a professional. A qualified technician can perform a full diagnosis and confirm the root cause before any parts are purchased. When in doubt, get a professional opinion — it can save you from an expensive misdiagnosis.
Real-world note
If the check engine light came on shortly after a fill-up, try the gas cap first before anything else. Many P0455 codes set because the cap wasn't fully tightened, was cross-threaded, or has a worn O-ring. A $15 replacement cap from an auto parts store often resolves it. If the code returns consistently after replacing the cap, the purge valve is the next most common culprit.
Learn more
How we write these guides
Find This Code guides are written based on OBD-II specification documentation, established automotive diagnostic practices, and real-world patterns observed across vehicle makes and models. Content is reviewed for accuracy and plain-English clarity before publication. Pages are marked with a “Last reviewed” date and updated when diagnostic guidance or code interpretation changes. All content is educational — it is not a substitute for hands-on diagnosis by a qualified mechanic.
Last reviewed May 2026 · Find This Code Editorial Team · Editorial standards →
Sources & references
Our guides are written based on OBD-II specifications, automotive engineering principles, and established diagnostic best practices. Key reference sources include:
- U.S. EPA — On-Board Diagnostics (OBD)Official EPA documentation on OBD-II standards and emissions monitoring requirements for passenger vehicles.
- ASE — Automotive Service ExcellenceIndustry certification body for automotive technicians; sets best-practice diagnostic and repair standards.
- California Air Resources Board — OBD ProgramCARB documentation on evaporative emission control system monitoring and leak detection thresholds.
Informational purposes only. This guide is for educational reference and is not a substitute for diagnosis by a qualified technician. Repair costs, causes, and symptoms may vary by vehicle make, model, year, and condition. Always consult a licensed mechanic before performing major repairs.