Trouble code P0442

P0442: small EVAP system leak.

P0442 is the generic OBD-II code “Evaporative Emission Control System Leak Detected (small leak)”. P0455 is the large-leak version.

Smell fuel? Stop.

Stop and see a professional, and don't keep driving, if you have a fuel leak or a smell of fuel.

What each part of the code means

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

How to read any trouble code.

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. California's OBD rules require the system to check itself for leaks as small as 0.040 inch and 0.020 inch across. On cars from the 2017 model year on, federal rules give the 0.020-inch leak its own “very small leak” code, such as P0456.

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

The gas cap: a cap that is loose or damaged and doesn't seal. Some cars warn about a loose cap separately, or set P0457 instead.
Hoses and fittings: cracked or split EVAP hoses, clamps that are loose or worn, or O-rings that have hardened.
The purge or vent valve: either one leaking or stuck open.
The charcoal canister: a crack or a split seam.
The filler neck or tank: a damaged or corroded filler neck, or a leaking seal on the tank or its sending unit.
The fuel tank pressure sensor: a faulty sensor can set the code too.

A shop can find the leak with a smoke test: smoke is pushed into the system, 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.
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.

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.
Give the leak test time to run again: it only runs when conditions set by the car's maker are met, so the EVAP monitor may not show ready straight after a repair. 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 P0442 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

P0455: large EVAP leak P0171: system too lean Check engine light on? Start here How to read a trouble code

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.