# P0456: Evaporative Emission System Leak Detected (very small 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:** Worn gas cap seal or tiny hose crack - **DIY?:** Steps 1-2 ## Quick answer P0456 means the engine computer has found a very small leak in the evaporative emission (EVAP) system, about 0.020 inch, smaller than a pinhole. Leaks this small are hard to find, so a gas cap with a worn seal and tiny cracks in hoses or valve seals are the usual suspects. It does not affect how the car drives. ## What does P0456 mean? The evaporative emission (EVAP) system traps gasoline vapor from the fuel tank in a charcoal canister instead of letting it escape into the air. When the engine runs, a purge valve lets the engine draw that stored vapor in and burn it. A vent valve at the canister closes during testing, so the whole system, from the gas cap to the purge valve, can be sealed and checked. To set P0456, the engine computer seals the system and watches the fuel tank pressure sensor to see whether pressure or vacuum holds. If it drifts by an amount that matches a leak of about 0.020 inch, roughly a pinhole, the test fails. Depending on the maker, the test runs with the engine idling, after the car is shut off, or after it has sat for hours, and only when fuel level and outside temperature are within set ranges. Most cars need the test to fail on two separate drive cycles before the light comes on. ## What are the symptoms of P0456? - Check engine light on - No change in how the car drives - No fuel smell in most cases - Failed emissions test ## What causes P0456? Ranked from most to least likely. Check them in this order: the cheap, common causes first. 1. **Worn gas cap seal** (most common). The rubber O-ring on the gas cap hardens, flattens, or cracks with age and heat, and a seal that looks fine can still pass a leak this small. It is the first suspect on any car, especially one more than several years old, and a cap left loose or cross-threaded after refueling causes the same result. 2. **Tiny crack in an EVAP hose or seal** (common). Rubber EVAP hoses and the O-rings at their plastic quick-connect fittings dry out and develop hairline cracks after years of engine heat and road salt. These leaks are too small to see or smell, so they usually show up only as smoke escaping during a smoke test. Older cars in hot or salty climates are affected most. 3. **Purge or vent valve not sealing fully** (common). The purge valve near the engine and the vent valve near the canister must seal completely during the leak test. Dirt, charcoal dust, or a worn valve seat can let a tiny amount of air slip past while the valve is closed. A valve that will not hold vacuum when tested on its own confirms it. 4. **Leaking fuel pump or sending unit seal** (less common). The fuel pump and fuel level sender sit in an opening on top of the tank, sealed by a large rubber ring under a locking collar. If that ring is reused, pinched, or not seated properly after fuel pump work, it can leak vapor. A P0456 that appears soon after a fuel pump replacement points here first. ## How do you diagnose P0456? 1. Replace or test the gas cap, clear the code, and see if it returns over the next week or two. 2. Have a smoke test done with a high-quality smoke machine, since very small leaks need careful inspection. 3. Check the purge and vent valves for leaks when closed. 4. Inspect the seal at the top of the fuel tank if the fuel pump was recently replaced. ## How do you fix P0456, and what does it cost? | Fix | Typical cost | What it involves | | --- | --- | --- | | Replace gas cap | $20-$50 | Use a quality cap made for your vehicle. | | Smoke test and replace leaking hose or seal | $150-$400 | Includes diagnostic time to find a tiny leak. | | Replace purge valve, vent valve, or canister | $150-$600 | Depends on which component leaks. | Full price breakdowns: [evap purge valve replacement cost](/cost/evap-purge-valve-replacement). ## Is it safe to drive with P0456? > **Safe to drive.** It is safe to drive. A very small EVAP leak is an emissions issue only. ## Will P0456 fail an emissions test? In most states that test OBD-II cars, a check engine light with P0456 stored is an automatic fail, even though the leak is tiny. Clearing the code right before the test does not help, because it also resets the readiness monitors, and the EVAP monitor is often the last one to finish. Some states also pressure-test the gas cap separately. ## After the repair: clearing P0456 Clear the code with a scan tool, or let it clear itself after several clean drive cycles. The EVAP monitor is slow: it usually needs a cold start after the car has sat for hours, often overnight, and a fuel level between about one-quarter and three-quarters. If the monitor shows complete and P0456 stays away for two weeks, the fix worked. ## How to prevent P0456 - Tighten the gas cap until it clicks after every fill-up, and keep it on its tether. - Replace a gas cap whose rubber seal is cracked, flattened, or hard with a quality cap made for your vehicle. - Stop filling at the first pump click, since topping off can push liquid fuel into the charcoal canister. - Ask for an EVAP smoke test after any fuel pump, fuel tank, or filler neck work. - On older cars in road-salt states, have a rusty filler neck checked, since rust can stop the cap from sealing. ## Codes often seen with P0456 - [P0442](/codes/p0442): Evaporative Emission System Leak Detected (small leak) - [P0455](/codes/p0455): Evaporative Emission System Leak Detected (large leak) - [P0440](/codes/p0440): Evaporative Emission System - [P0441](/codes/p0441): Evaporative Emission System Incorrect Purge Flow ## Frequently asked questions ### Can I drive with a P0456 code? Yes, P0456 has no effect on how the car drives, accelerates, or uses fuel. It is an emissions fault that lets a little gasoline vapor escape. You only need to fix it to turn off the check engine light and pass an emissions test, but it is worth fixing within a few weeks. ### Why is P0456 so hard to find? Because the leak is only about 0.020 inch across, it rarely shows up on a visual check and usually gives off no fuel smell. Shops use a smoke machine that fills the EVAP system with visible vapor, and a very small leak may need a slow, careful test or even a second visit to spot. ### Should I replace the gas cap for P0456? Yes, a new cap is a sensible first step because it costs about $20 to $50 and often fixes it. Buy a quality cap made for your vehicle, since cheap universal caps may not seal well. If the code returns after one to two weeks of normal driving, look for a leak elsewhere. ### What is the difference between P0456 and P0442? Both mean the EVAP system has a leak, but they flag different sizes. P0442 is a small leak, usually around 0.040 inch, while P0456 is a very small leak of about 0.020 inch. The causes and tests are similar, but P0456 leaks are harder to find because they are so tiny. ### How much does it cost to fix P0456? Most P0456 repairs cost $20 to $600. A gas cap is $20 to $50, a smoke test plus a hose or seal is about $150 to $400, and a new purge valve, vent valve, or charcoal canister runs $150 to $600, depending on which part leaks and how hard it is to reach. ### Will P0456 go away on its own? Sometimes, if the cause was a loose gas cap that has since been tightened. The light turns off by itself after several drive cycles in which the EVAP test passes. If the leak is real, such as a cracked hose, the code will come back each time the test runs and fails again. ### Will a P0456 code fail an emissions test? Yes, in most states that test OBD-II cars, a lit check engine light with a stored code fails the test, even for a tiny EVAP leak. Clearing the code just before the test does not help, because that also resets the readiness monitors. Repair it first, then drive normally for a week or two. ### Can I fix P0456 myself? You can handle the easy steps yourself: replace the gas cap, make sure it clicks, and look over the hoses you can reach near the canister and engine. Finding a leak this small usually takes a smoke machine, so most drivers hand the car to a shop if a new cap does not fix it. ### Can overfilling the gas tank cause P0456? It can contribute, though it is not the usual cause. Topping off after the pump clicks can push liquid fuel into the charcoal canister and vent valve, which can make the vent valve stick or seal poorly. Over time that can lead to EVAP leak codes, so stop filling at the first click. ### Does cold weather make P0456 show up? It can, because the EVAP test only runs when outside temperature and fuel level are within ranges that vary by maker. Rubber seals also shrink and stiffen in the cold, so a marginal gas cap or hose seal may leak in winter and seal again in warmer months. So the code can come and go. ## Sources - [SAE J2012: Diagnostic Trouble Code Definitions](https://www.sae.org/standards/content/j2012_201812/) ## 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) - [P0455 Evaporative Emission System Leak Detected (large leak)](https://diagxcar.com/codes/p0455) - [P0457 Evaporative Emission System Leak Detected (fuel cap loose/off)](https://diagxcar.com/codes/p0457) ## 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 EVAP system?](https://diagxcar.com/glossary/evap-system) --- Source page: https://diagxcar.com/codes/p0456 Updated: 2026-11-03 General information, not a substitute for a qualified mechanic.