P0455 code: large EVAP system leak

What each part of the code means

P: Powertrain
0: Generic code, defined by SAE
4: Auxiliary emissions controls
55: The specific fault

How serious is it?

DeepDiag rates a stored P0442 or P0455 as minor, and a pending one as pending. The rating is a starting point, not a safety check.

Possible causes

The evaporative emission (EVAP) system catches fuel vapor from the tank, holds it in a charcoal canister, and later draws it into the engine to be burned. To check itself for leaks, the system seals itself off, then pulls a vacuum or builds pressure inside, using engine vacuum on some cars and a small pump on others. P0455 sets when the test can't reach its target, or the vacuum bleeds away almost at once.

California's OBD rules require tests for leaks of 0.040 and 0.020 inch, but they don't define a “large” leak. Makers judge it by how the test behaves, for example how quickly the vacuum is lost, rather than by the size of the hole.

In no particular order: which one is likeliest depends on your car and its data.

The gas cap: a cap that's missing, loose or not tightened until it clicks, the wrong type for the car, or one whose seal has cracked or hardened. If the light appeared a day or two after you filled up, check the cap first.
Hoses and fittings: an EVAP hose that has split, cracked or rubbed through, or a connector that has come off.
The vent valve: a vent valve that sticks open, or that dirt keeps from closing, leaves the system open to the air, so the test can't seal it.
The purge valve: on systems that use engine vacuum for the test, a purge valve that won't open stops the computer from pulling a vacuum, and the test reads that as a large leak.
The charcoal canister: a crack or a split.
The filler neck or tank: a filler neck that is damaged or rusted where the cap seals, or a leaking seal on the tank, the fuel pump or the sending unit.
The fuel tank pressure sensor: a faulty sensor can set the code even when there is no leak.

A shop can find the leak with a smoke test: smoke is pushed into the sealed system at low pressure, and the leak shows where it comes out.

What to check in the data

EVAP monitor tests: the 0.040-inch and 0.020-inch leak tests and the purge flow test, where the car reports them.
Purge and vapor pressure: live commanded evaporative purge and EVAP system vapor pressure, where the car supports them.
Fuel level: live fuel tank level, where the car supports it.
Emissions readiness: whether the evaporative system monitor has completed its self-test since codes were last cleared.
Freeze frame: the sensor snapshot from the moment the fault was recorded.
On-board leak test: DeepDiag can request an EVAP leak test, and asks you to confirm first because the command moves real vehicle components. Most cars don't support these tests, and the app tells you when yours doesn't.
Other codes: a failure in one component can set a code in another, so read every stored and pending code.

Before you clear it or buy parts

Clearing erases stored and pending codes and resets the emissions readiness checks, so the car may fail an emissions test until it has been driven enough for them to complete again. If a stored code comes straight back, that points to an active fault.

Rank the possible causes by likelihood, then run the simplest, cheapest test that would confirm the cause before you buy a part. Read what to do before you clear codes.

Tell the shop if you've touched the cap: if a shop will test for the leak, say whether you tightened or replaced the cap. Doing that before the test can hide whether the cap was the leak.
A new cap only fixes a cap leak: if the leak is somewhere else, the code comes back. Check the filler neck too: no cap can seal against a neck that is rusted or damaged where the cap sits.
Test valves before replacing them: GM says purge solenoids and pumps sent back under its warranty showed no fault when tested, and advises replacing them only when diagnosis points to them.
Don't top off the tank: squeezing in extra fuel after the nozzle shuts off can push liquid fuel into the EVAP system and damage the charcoal canister.
Give the leak test time to run again: it only runs when conditions set by the car's maker are met, and California's rules turn the light off once the test has run and passed on three drives in a row. New York's inspection program gives these as general conditions: a tank between a quarter and three-quarters full, an outside temperature between 30 and 95°F, and a car that has sat for at least 8 hours. California's rules also let makers skip the test when the tank is more than 85% full.

How DeepDiag helps

DeepDiag reads P0455 and the data above, then explains in plain language what the data shows, what the AI suspects and how confident it is. It also suggests checks to run before you buy parts, and says whether it's a job you can do yourself.

DeepDiag gives an AI-assisted reading of the data your car reports. It isn't a professional inspection, a safety check or a guarantee, and the AI can be wrong. Every feature needs an active Premium subscription.

Coming soon

Get told when it's ready See a sample report

Sources

The possible causes, definitions and repair tips on this page come from these sources, checked in November 2026. What DeepDiag reads and how it rates a code come from the app itself.

P0455: large EVAP system leak.

Smell fuel? Stop.