P0456 Code: Very Small EVAP Leak and Will It Clear Itself?

P0456 names the size of a leak, not where it is. That's why a new gas cap fixes it on one car and changes nothing on the next.

The filler, where a P0456 check starts

What does a P0456 code mean?

P0456 means your car's OBD-II leak test found a very small leak in the fuel vapor system, about 0.020 inches across. That's roughly the width of a pencil lead, the smallest leak the federal rule makes a car find. OBD-II is the self-diagnosis system every car since 1996 carries.

The system catches gasoline fumes from the tank, holds them in a charcoal canister and feeds them to the engine to burn. A hole in it changes nothing you can feel or hear, so the code is an emissions failure, not a breakdown warning.

You'll also see it written PO456, with a letter O, but the second character is a zero. Every scanner and repair order prints it that way.

Can you drive with a P0456 code?

Yes, if the car runs normally, because a leak in the vapor system doesn't change how the engine runs. Plan the repair, not a tow. On a 2012–2018 Focus 2.0L, run the VIN first, since a purge valve recall there covers a stall while driving.

The exception is a stuck-open purge valve, the valve that lets the stored fumes into the engine. One carmaker says a failed one can bring hesitation, rough running or stalling at low speed, so have a car that stumbles checked before you drive far, and tow one that stalls.

The leak lets vapor escape slowly, and the car is otherwise sound. A strong gasoline smell is different: with that, liquid fuel under the car or stalling, stop, keep flames and sparks away, and have it towed.

A flashing check engine light is a different fault, because an evaporative fault doesn't flash. Our flashing check engine light guide covers that one.

Will a P0456 code clear itself?

Yes, a P0456 check engine light can go out by itself after three trips in a row in which the leak test runs and finds nothing. The stored code stays longer.

The stored code can take 40 warm-ups to erase, drives that bring the engine up to temperature, without the fault. If a loose or missing cap caused it, the lamp may go out once the car verifies the cap is secured.

Clearing the code with a scan tool doesn't erase everything, because the car also keeps a permanent copy. That copy goes only when the car's own check runs again and finds no fault, and none of this means the leak has gone.

A lamp that went out the same day as a repair was most likely cleared by the shop. Unless the cap was the cause, it normally needs three clean trips. So have the codes read even after the light has gone, because a dark dash is the weakest evidence on the car.

Can a loose gas cap cause a P0456?

Yes, and the rule has a clause for it: once the car verifies the cap is secure, the lamp may go out. Take the cap off, check the seal and put it back until it clicks.

If the light stays on after a few drives with the cap tight, the leak is somewhere else, while a P0457 beside it points at the cap itself. A capless filler has no cap to check, so the smoke test covers it.

Will P0456 fail an emissions test?

Yes, where the test reads the car's computer, so fix it first and drive until the leak test reads ready. A car whose codes were just cleared can be turned away too, since its self-tests haven't finished.

Some states let a newer car pass with one self-test still unfinished, so ask your test station before you go. The leak test is often the last to finish, because it needs a cold start and a steady drive in mild weather before it runs.

What causes a P0456 code?

Any part of the sealed vapor system with a hole about the width of a pencil lead can set it. That means the cap or a capless filler (a sprung flap with no cap), the vent valve that lets the canister breathe, the purge valve, the hoses, the canister, and on some cars a leak detection switch.

Those causes run cheapest first. P0456 alone confirms none of them, because the smoke test, not the code, finds the hole.

How do you fix a P0456 code?

Find the hole first, then replace only the part it's in. A shop fills the sealed system with low-pressure smoke and watches where it escapes, the one test that reliably finds a leak this small.

Fixing a P0456 code starts at the gas cap and ends at the canister, cheapest first:

1. The gas cap. Take it off, inspect the seal and put it back until it clicks. A capless car has no cap to check, so the smoke test covers its filler instead.
2. The vent valve or the leak detection switch. They sit under the car at the charcoal canister.
3. The purge valve. It sits under the hood. Run the VIN first, because some carmakers pay for one: our free VIN checker lists your model year's recalls, NHTSA's VIN search shows which are open, and a dealer can tell you about warranty programs.
4. Hoses and the canister. The expensive end, so a smoke test should find the leak there before anyone orders a canister.

Ask for the smoke test by name, and ask what it costs before the parts conversation starts. A quote that names a part before anyone has smoked the system is a guess with an invoice attached.

Then ask whether the leak monitor, the car's own leak test, has run and passed on a scan tool, since it needs certain driving conditions to finish. FCA made that test mandatory for its dealers, because without it nobody knows whether the repair worked.

How much does it cost to fix a P0456 code?

Often nothing, if the cap was loose, and a new cap is the cheapest part the code can point to. Past the cap, the smoke test names the part, and a revised vent valve on a 2015 Silverado 1500 5.3L costs about $185–$342, parts and labor, at an independent shop.

The vapor canister is the priciest part, and the federal emission warranty covers parts like these for two years or 24,000 miles from delivery, longer in states that follow California's rules. Some carmakers also run programs that pay for a purge valve or a canister, so ask the dealer before you pay a shop.

At the counter, ask for the smoke test result, the hours and the rate on the written quote. Ask too whether the test fee comes off if you approve the repair.

Why does P0456 keep coming back?

Because the car doesn't run the small-leak test on every drive. The carmaker sets the timing, and may skip the test above 85 percent of the tank or run it only after a cold start. The lamp waits for a second failed test, and for a leak this small a carmaker may take up to 12 drives on average.

One carmaker tells its dealers that the drive cycles that set P0456 can't be repeated during a dealer visit. So a car in a cold snap, a heatwave or on short full-tank trips can go many drives without a test that counts.

Why did the repair look like it worked?

Because clearing the codes turns the lamp off that minute, while the car needs days of the right weather before it tests the repair. The proof can come days after the repair.

When the leak test finally runs, it can find the leak that was there all along, somewhere else. The lamp going out at the counter proves the codes were cleared, and nothing about the leak.

So before paying for a second part, ask whether the leak test has run and passed since the first one. A scan tool shows whether that test has finished, called readiness, and it's a different question from whether a code is stored.

P0456 vs P0455, P0442 and P0457: are they the same test?

On many cars, yes: the federal emissions rule asks the car to find leaks at 0.040 and 0.020 inches, and the code says which it found. P0455 is the large end, a size the rule leaves to the carmaker.

P0457 is a different kind of code: the car traced the leak to a loose or missing cap, so start there. A 0.020-inch hole elsewhere is usually too small to see, hear or smell, which is why the smoke test finds it.

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Which codes come with P0456?

P0455, P0441 and P0442 come with it most in carmaker bulletins. Two leak codes together still point to one leak. Here's what each code beside P0456 tells you about where to look:

• P0455 or P0442. The same leak test at a bigger size, so the search stays on the hole.
• P0457. The car traced the leak to a loose or missing cap, so start there.
• P0446 or P0449. The vent valve or its circuit, at the canister.
• P0441 or P0496. Purge flow failed as well, so the search moves to the purge side.
• P0443. The purge valve's own circuit failed its electrical check.

Does P0456 point to a different part on each make?

The leak test works the same way on every car, but each carmaker builds the vapor system its own way. So the code stays the same while the first suspect changes.

P0456 on Jeep, Dodge and Ford

One module at the charcoal canister, called the ESIM, runs the leak test on Jeep, Dodge, Ram and Chrysler models. When the fault is in that module, Stellantis tells its dealers to replace only the module and leave the canister alone.

Carmaker bulletins also record factory faults behind P0456, and none is a worn part. A vapor hose can leave the factory unplugged, with a fuel smell at one wheel, and on other models the fix is a software update.

Whatever the repair, ask the dealer to run the leak test on a scan tool before and after it. A road test can't confirm the fix, because the code only sets over several cold starts and drives.

On a Ford, the documents point first at the purge valve under the hood. A valve stuck open can pull the tank into a vacuum, and Ford's recall says that can deform the tank.

Run the VIN before you pay for a valve, because some Ford models have a recall or a longer warranty for it. Before any EVAP part is replaced, ask for the full leak test and the purge valve test, as a Ford dealer notice does.

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Common questions

Asked most often

Is a P0456 code a DIY fix?

Partly, because checking the gas cap and its seal costs nothing. The smoke test that finds a 0.020-inch leak is a shop job, and so is running the leak monitor on a scan tool.

What happens if I leave a P0456 code?

Nothing you can feel if the car runs normally, but it fails an emissions test that reads the computer. A purge valve stuck open is the exception, because it can make the engine stumble or stall.

Is a P0456 repair covered by warranty?

Often, if the car is young: the federal emission warranty covers evaporative parts for two years or 24,000 miles from delivery, longer in states that follow California's rules. Some carmakers also run programs that pay for a purge valve or a canister, so ask a dealer before you pay.

This article is general information, not advice about your specific vehicle. See the disclaimer.