P0442 — Evaporative Emission System Leak Detected (Small Leak)
P0442 is the small-leak version of P0455. The car drives normally and nothing is at risk, but the leak is too small to find by eye — this is the code that needs a smoke machine rather than a careful look.
The system plays no part in running the engine, so the vehicle drives normally. It will fail an emissions test until the leak is sealed and the monitor re-runs. This code will cause an emissions test failure.
What P0442 means
The module seals the evaporative system and watches how quickly it loses pressure or vacuum. A slow loss is reported as a small leak, which means an opening roughly the size of a pinhole rather than a missing cap seal. That size distinction is the whole diagnosis: a large leak is usually visible, while a small one is typically a hairline crack in a plastic line, a perished seal, or a vent valve not quite closing, and inspection alone will rarely find it.
| System | Emission control |
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Symptoms you may notice
What causes P0442
Hairline crack in a vapour line
The plastic lines harden with age and crack at bends and clips, often opening only when warm. This is the classic small-leak cause and the reason a smoke test exists.
Fuel cap seal hardened
The rubber seal flattens and hardens with heat cycling. It still holds the cap on and still looks intact, but no longer seals to the standard the small-leak test applies. Cheapest cause and worth eliminating first even though it is less likely here than with a large leak.
Vent valve not closing fully
Debris or corrosion on the vent valve seat leaves a small path open during the test. The valve is mounted underneath where road salt and water reach it, so this is common on older vehicles in wet climates.
Charcoal canister seal leaking
A cracked canister housing or a perished grommet where a line enters it produces exactly this signature. Less common, and usually found by the smoke test rather than suspected first.
How to diagnose it
Inspect and reseat the fuel cap
Look closely at the seal for flat spots and hardening. Refit until it clicks, clear the code, and drive normally for several days so the monitor can re-run.
Smoke test the system
Introduce low-pressure smoke through the service port and watch every line, joint and the canister. A small leak will rarely be found any other way.
Command the vent valve closed and re-test
Close the vent valve with the scanner and repeat the pressure test. A valve that does not seal will show as a leak that disappears when the valve is held shut manually.
Inspect the filler neck and tank seal
With the vehicle safely supported, check where the filler neck meets the tank and where the sender unit seals. Corrosion at the neck is a common small-leak source on older vehicles.
Safety
Never work under a vehicle supported only by a jack, and keep all ignition sources away from the fuel system.
Related codes
Common questions
What is the difference between P0442 and P0455?
Only the size of the leak. P0455 is a large leak, usually an unsealed cap, and often visible. P0442 is a small one — closer to a pinhole — and normally needs a smoke test to locate.
Is it safe to drive?
Yes. The evaporative system has no role in running the engine. You will fail an emissions test and may occasionally smell fuel.
Why can I not see the leak?
A small leak is by definition too small to see. Cracks that only open when the plastic is warm are common, which is why smoke testing beats visual inspection here.
Last verified 2026-07-27 · Sources: SAE J2012 — Diagnostic Trouble Code Definitions and Failure Type Byte (SAE International)
This information is for general guidance and is not a substitute for professional diagnosis. Cost figures are estimates, not quotes.