P0455 and P0442 (EVAP leak): is it just the gas cap?

What do P0455 and P0442 mean?

Both codes come from the evaporative emission (EVAP) system, which keeps gasoline vapor from escaping the fuel tank. Nissan describes the basic flow in its EVAP bulletin (Nissan NTB13-097g):

• Fuel vapor flows from the fuel tank into a canister.
• Vapor flows from the canister through a purge control valve to the engine.
• Fresh air flows into the canister through a vent.

The codes say how big a leak the car found. Manufacturers word them slightly differently:

Sources: GM 21-NA-124, Mazda 01-002/18. Honda also calls P0456 a "very small leak" (Honda 14-010).

How does the car test for a leak?

California's OBD-II regulation requires the car to check that purge flow reaches the engine, and to check the whole EVAP system for vapor leaks (13 CCR § 1968.2, section (e)(4)). It defines the leaks by the size of a precision test hole:

• a leak (or leaks) equal to a 1.0 mm (0.040 in) diameter hole;
• a leak equal to a 0.5 mm (0.020 in) diameter hole.

A manufacturer may get approval to use a 2.3 mm (0.090 in) hole instead of the larger size. The regulation doesn't assign code numbers to these sizes. Our reading of the code names above: P0442 is usually the larger of the two required sizes and P0456 the smaller, while P0455 covers something bigger, such as a missing cap or a disconnected hose. Your car's service information is the authority.

The test doesn't run all the time. Under the regulation, a manufacturer may:

• skip or stop the test when the tank is more than 85% full, or while you refuel;
• run it only after a cold start, with approval;
• with approval, use a statistical method that needs up to twelve drives on average to confirm the smallest leak.

That is why an EVAP code can take days to set, and days to clear (our inference). Nissan even tells its technicians that a test drive isn't part of confirming these codes and "will only waste time"; they pressure-test the system instead.

Scope. The regulation covers 2004 and newer vehicles certified for California. The California Air Resources Board (CARB) says virtually all vehicles sold in the US are designed and certified to its OBD II requirements. We haven't checked other countries' rules.

Is it just the gas cap?

Often it's worth checking first, and sometimes that's all it is:

• Honda (2019 Fit owner's guide) says the check-engine light can mean "the fuel cap is missing or loose".
• Nissan (2012 and newer models, except the LEAF and the Titan XD diesel) says that if a cap is left loose for several trips, leak codes are stored and the light comes on; P0442, P0455 or P0456 "may" be a loose cap. When the leak test then finds no leak, it tells technicians to erase the codes and "DO NOT REPLACE FUEL FILLER CAP OR ANY OTHER EVAP RELATED PARTS" (Nissan NTB13-097g).
• Honda found that on many 2012 models and some 2011 and early-2013 models (Accord, Civic, CR-V, Fit, Pilot and others, listed by VIN) the cap's gasket created too much friction for the cap to seal, setting P0455 or P0456. The cap might feel loose or have a "stick-slip feeling" when tightened; technicians inspect the cap and replace it if necessary (Honda 14-010).
• Capless fillers have their own version. GM's 2025 Equinox manual says that if you refueled from a gas can with the funnel adapter, make sure it has been removed. The system can detect an adapter left in place, and a few trips without it may turn the light off.

The regulation allows for the cap case too. If the car can tell that the leak is a loose or missing cap, it may use a separate warning instead of the check-engine light. Or, if the check-engine light came on, it may turn it off once it has verified that the cap is secure and nothing else is wrong (13 CCR § 1968.2, (e)(4.4.2)).

What else causes P0455 and P0442?

GM's diagnostic tip for its 2000–2021 gasoline vehicles lists what technicians find (GM 21-NA-124):

GM names seven connections to check, from the purge valve at the engine back to the canister, the vent valve, the fuel tank lines and "fuel tank cap is closed properly". It notes that some leaks show only under pressure and others only under vacuum. Mazda adds that, on its 2003–2015 models covered by the bulletin, the purge-flow code P0441 can also be stored when the filler cap wasn't closed properly after refueling (Mazda 01-002/18).

What should you do?

1. Check the cap. Take it off, look at the seal for cracks or damage, refit it the way your owner's manual says, and make sure nothing is caught in the filler. On a capless filler, make sure no funnel adapter is left in.
2. Give it a few days of normal driving. The light may go out on its own once the test runs and passes. Don't overfill the tank: GM warns that overfilling "by more than three clicks of a standard fill nozzle" can cause stalling, fuel-system damage, spills and, in some conditions, fuel fires.
3. If the light stays on or comes back, get the system tested. GM and Nissan technicians use a smoke machine or a pressure tester to find the leak. Replacing parts on a guess is what Nissan tells its dealers not to do.
4. Read pending codes as well as stored ones. An EVAP fault can show up as a pending code first. See pending, stored and permanent codes.
5. Don't clear the code right before an emissions test. The EVAP test has to run again, and until it does the car may read "not ready". See why a car says "not ready".

Can you keep driving?

Usually, yes, if the car runs normally. For a steady check-engine light, CARB says most manufacturers advise service "as soon as conveniently possible" (CARB). If your car shows a loose-fuel-cap warning, Nissan's advice is to stop safely and make sure the cap is on correctly.

If you smell fuel, stop. Park away from buildings, switch the engine off, get everyone out and don't restart until it has been checked. GM's manual warns that fuel vapors and fuel fires "burn violently". The same goes for fuel dripping under the car (our inference: a vapor leak code can't tell you whether liquid fuel is leaking too).

If the light flashes or the engine runs badly, the EVAP code isn't the whole story. See Check engine light on: can I keep driving?