# P0455: Evaporative Emission System Leak Detected (large leak) **Verdict:** Safe to drive Emissions - Generic OBD-II code, applies to all gasoline cars and light trucks sold in the US since 1996. - **Typical cost:** $20-$600 - **Urgency:** Keep driving - **Most likely:** Missing, loose, or bad gas cap - **DIY?:** Yes, check the cap first ## Quick answer P0455 means the engine computer has found a large leak in the evaporative emission (EVAP) system, the sealed system that stores fuel vapor from the gas tank. "Large" means the system could not hold vacuum or pressure at all during its self-test. A missing or loose gas cap is the top cause, but a disconnected hose or a stuck-open purge or vent valve can also set it. ## What does P0455 mean? The EVAP system holds gasoline vapor from the tank in a charcoal canister until the engine can burn it. To make sure vapor is not escaping, the engine computer seals the system by closing the vent valve and then tries to draw a vacuum or build pressure using engine vacuum, a small pump, or the natural pressure change as the tank cools. A fuel tank pressure sensor, or a reading from the leak-test pump, tells it how well the system holds. P0455 sets when the system cannot build or hold any meaningful vacuum or pressure at all, which the computer treats as a large or gross leak. That usually means an opening much bigger than a pinhole, such as a missing cap or a hose that has come off. Some vehicles run part of this check soon after refueling, which is why a loose cap often turns on the light within a day or two. Most cars need two failed tests before the check engine light comes on. ## What are the symptoms of P0455? - Check engine light on, often right after refueling - Fuel smell near the car - Car drives normally - Failed emissions test ## What causes P0455? Ranked from most to least likely. Check them in this order: the cheap, common causes first. 1. **Missing, loose, or damaged gas cap** (most common). Forgetting to tighten the cap after filling up is the classic cause of P0455, and the light often appears within a day or two of refueling. A cracked or missing seal, a broken cap, or a cheap cap that does not fit does the same thing. On capless fillers, a stuck or dirty flap can leak. 2. **Disconnected or split EVAP hose** (common). A hose that has popped off its fitting or split badly lets the system leak freely, so it cannot hold any vacuum. This often happens after other repairs nearby, such as intake or fuel pump work, or when old plastic connectors break. Large leaks like this usually show up fast on a smoke test. 3. **Purge or vent valve stuck open** (common). If the purge valve sticks open, the system is open to the engine intake; if the vent valve sticks open, it is open to the air. Either way it cannot seal for its leak test and may read as a large leak. A purge valve stuck open can also cause hard starting right after refueling. 4. **Cracked charcoal canister or leaking filler neck** (less common). Physical damage from road debris, an impact, or rust can open a large leak in the charcoal canister or the filler neck. Rusted filler necks are more common on older vehicles in states that salt their roads. These parts usually sit near the tank, out of sight, and need replacement rather than repair. ## How do you diagnose P0455? 1. Check that the gas cap is present, tightened until it clicks, and has an intact seal. 2. Look under the car around the tank and canister for disconnected hoses. 3. Have a smoke test done if the cap is good. Large leaks usually show smoke quickly. 4. Command the purge and vent valves to confirm they open and close. ## How do you fix P0455, and what does it cost? | Fix | Typical cost | What it involves | | --- | --- | --- | | Tighten or replace gas cap | $20-$50 | Fixes most P0455 codes. | | Reconnect or replace EVAP hose | $80-$350 | Includes finding the leak with a smoke test. | | Replace vent valve, purge valve, or canister | $150-$600 | For a stuck valve or cracked canister. | Full price breakdowns: [evap purge valve replacement cost](/cost/evap-purge-valve-replacement). ## Is it safe to drive with P0455? > **Safe to drive.** It is safe to drive, but a strong fuel smell means fuel vapor is escaping and should be checked promptly. ## Will P0455 fail an emissions test? P0455 with the check engine light on fails an OBD-based emissions test in most states. Some state programs also pressure-test the gas cap on its own. If you just tightened or replaced the cap, do not clear the code right before testing, because the EVAP monitor will read not ready. Drive for several days first. ## After the repair: clearing P0455 After tightening or replacing the cap or fixing a hose, clear the code or wait for the light to go off on its own after several days of normal driving. The leak test needs the right fuel level and temperatures, often after an overnight cool-down. The repair worked when the EVAP monitor shows ready and no pending code returns. ## How to prevent P0455 - Tighten the gas cap until it clicks every time, and never drive off with it on the roof. - Replace a gas cap whose seal is cracked or whose tether is broken. - Use an original-equipment or quality replacement cap designed for your vehicle. - Look for loose EVAP hoses after any work near the tank, canister, or engine intake. - Stop at the first pump click instead of topping off, which can damage the canister. ## Codes often seen with P0455 - [P0442](/codes/p0442): Evaporative Emission System Leak Detected (small leak) - [P0456](/codes/p0456): Evaporative Emission System Leak Detected (very small leak) - [P0440](/codes/p0440): Evaporative Emission System - [P0446](/codes/p0446): Evaporative Emission System Vent Control Circuit - [P0441](/codes/p0441): Evaporative Emission System Incorrect Purge Flow ## Frequently asked questions ### Can a loose gas cap cause P0455? Yes, a loose or missing gas cap is the most common cause of P0455. Tighten it until it clicks, and the light should go off on its own after several days of driving if that was the problem. If the seal is cracked or the cap is damaged, replace it. ### Can I drive with a P0455 code? Yes, P0455 does not harm the engine or change how the car drives. Fix it before an emissions test and sooner if you smell fuel, since a large leak can let a lot of vapor escape. If the cause is the gas cap, the fix may cost nothing at all. ### How long after tightening the gas cap will the light go off? Usually within a few days to a week of normal driving. The computer must rerun its leak test, which only happens under certain temperature and fuel-level conditions, often after an overnight cool-down. If the light is still on after a week or so, the leak is probably somewhere else. ### How much does it cost to fix P0455? Most P0455 repairs cost $20 to $600 at an independent shop. Tightening the cap is free and a new cap is $20 to $50. Reconnecting or replacing an EVAP hose, including a smoke test, runs $80 to $350, and a vent valve, purge valve, or canister costs $150 to $600. ### Will P0455 make me fail an emissions test? Yes, P0455 will fail an OBD-based emissions test in most states while the check engine light is on. Clearing it right before the test resets the EVAP monitor, which then shows not ready. Fix the leak and drive several days so the monitor can complete. ### Can I fix P0455 myself? Often yes, because the most common cause is the gas cap, which anyone can tighten or replace. You can also look under the car near the tank and canister for a hose that has come off. If those look fine, a shop smoke test is the quickest way to find the leak. ### What is the difference between P0455, P0442, and P0456? All three are EVAP leak codes that differ by leak size. P0455 is a large leak, P0442 is a small leak of about 0.040 inch, and P0456 is a very small leak of about 0.020 inch. Large leaks are usually the easiest to find, often at the gas cap. ### Can a bad purge valve cause P0455? Yes, a purge valve stuck open can cause P0455 because the system stays open to the engine and cannot seal for its leak test. Hard starting or a rough idle right after refueling is a common clue. Testing the valve with the engine off shows whether it holds closed. ### Is a P0455 fuel smell dangerous? A faint fuel smell around the cap is mostly an emissions issue, but a strong or growing smell is a safety concern. Fuel vapor is flammable, so check for liquid fuel under the car and keep sparks and flames away. Have a strong smell inspected promptly rather than waiting for an emissions test. ### Why did P0455 come on right after I got gas? It usually means the gas cap was not tightened fully or the seal did not seat after refueling. Some vehicles also show a check fuel cap message for this reason. Remove the cap, check the seal for dirt or damage, and tighten it until it clicks, then drive normally. ## Sources - [SAE J2012: Diagnostic Trouble Code Definitions](https://www.sae.org/standards/content/j2012_201812/) - [EPA: On-Board Diagnostics (OBD)](https://www.epa.gov/state-and-local-transportation/vehicle-emissions-board-diagnostics-obd) ## Other emissions codes - [P0440 Evaporative Emission System](https://diagxcar.com/codes/p0440) - [P0441 Evaporative Emission System Incorrect Purge Flow](https://diagxcar.com/codes/p0441) - [P0442 Evaporative Emission System Leak Detected (small leak)](https://diagxcar.com/codes/p0442) - [P0443 Evaporative Emission System Purge Control Valve "A" Circuit](https://diagxcar.com/codes/p0443) - [P0446 Evaporative Emission System Vent Control Circuit](https://diagxcar.com/codes/p0446) - [P0449 Evaporative Emission System Vent Valve/Solenoid Circuit](https://diagxcar.com/codes/p0449) - [P0451 Evaporative Emission System Pressure Sensor/Switch "A" Circuit Range/Performance](https://diagxcar.com/codes/p0451) - [P0452 Evaporative Emission System Pressure Sensor/Switch "A" Circuit Low](https://diagxcar.com/codes/p0452) - [P0456 Evaporative Emission System Leak Detected (very small leak)](https://diagxcar.com/codes/p0456) - [P0457 Evaporative Emission System Leak Detected (fuel cap loose/off)](https://diagxcar.com/codes/p0457) - [P0496 Evaporative Emission System High Purge Flow](https://diagxcar.com/codes/p0496) - [P0498 Evaporative Emission System Vent Valve Control Circuit Low](https://diagxcar.com/codes/p0498) ## Symptoms - [Why does my car smell like gas?](https://diagxcar.com/symptoms/car-smells-like-gas) ## Repair costs - [EVAP purge valve replacement cost](https://diagxcar.com/cost/evap-purge-valve-replacement): $120-$350 ## Warning lights linked to this - [Check engine light](https://diagxcar.com/warning-lights/check-engine) ## Terms explained - [What is Check engine light?](https://diagxcar.com/glossary/check-engine-light) - [What is EVAP system?](https://diagxcar.com/glossary/evap-system) --- Source page: https://diagxcar.com/codes/p0455 Updated: 2026-11-03 General information, not a substitute for a qualified mechanic.