P0442

00:00What this code means

Code P0442. Evaporative Emission System Leak Detected (Small Leak).

In plain English: the fuel-vapour system pulled into vacuum but could not hold it, which points to a pinhole or marginal seal rather than something wide open.

This normally will not change how the car drives or leave you stranded. It will fail an emissions inspection and it allows fuel vapour to escape. A strong raw-fuel smell or visible liquid fuel is not a small diagnostic leak; park safely, shut the engine off and inspect it promptly.

00:35How the test actually works

Here's what your engine computer is actually measuring.

Fuel vapour is supposed to stay inside a sealed path from the tank and filler cap to the charcoal canister. The vent valve closes the path to atmosphere and the purge valve or a leak-detection pump creates a small pressure difference. A pressure sensor then watches whether the sealed system holds.

The exact calibrated leak size varies by vehicle. P0442 means the system created enough vacuum or pressure to run the test, then decayed faster than the small-leak limit. That slow loss points toward a hard cap seal, a hairline hose crack or a valve that seeps, not the disconnected hose that usually produces P0455.

So the code is not a part failure report. It's a performance measurement that came back under spec.

01:27Symptoms

What you'll usually notice, beyond the light itself.

Check engine light with normal driving. The usual presentation. The engine often feels completely normal.

Failed emissions or smog inspection. An EVAP monitor fault is enough to fail even when tailpipe emissions feel irrelevant.

Faint fuel-vapour smell after refuelling. Often strongest near the cap, filler neck or charcoal canister.

The light comes and goes with temperature. A marginal rubber seal may pass warm and leak cold, or the other way around.

No visible leak anywhere. That is normal for P0442. A small vapour leak is usually found with pressure or smoke, not by looking for liquid.

02:19Most likely causes, ranked

Now the part you came for: what actually causes this, ranked by how often it turns out to be the culprit.

28 percent of the time, it's this: Fuel-cap seal hardened, cracked or contaminated. Still the first check, but small leaks are more often a marginal seal than a cap left completely loose.

24 percent of the time, it's this: Hairline crack in an EVAP hose or connector. Most often at a bend, quick connector or the short rubber sections near the canister.

17 percent of the time, it's this: Purge valve leaking when commanded closed. A valve can seal well enough to avoid P0455 yet seep enough to fail the small-leak test.

13 percent of the time, it's this: Vent valve or vent-valve seal leaking. Mounted underneath where dust, water and road salt attack the sealing surface.

10 percent of the time, it's this: Hairline crack in the charcoal canister. Usually from impact, corrosion or liquid fuel saturation after repeated overfilling.

8 percent of the time, it's this: Filler-neck, pump-module or tank seal. Hard to see without an approved low-pressure smoke test and often located above the tank.

Notice that the most expensive repair is not the most likely one. That matters, because the cheap checks come first.

03:46The measurement that decides it

This is the single measurement that tells you whether you need the expensive part or the cheap one.

Watch EVAP pressure during the vehicle's self-test, or use an approved EVAP test machine and the factory procedure. The system uses very low pressure because fuel vapour is present; never improvise with shop air or oxygen.

A sealed system pulls down to the test level and holds nearly flat. A tiny amount of drift is normal, but it stays comfortably inside the pass threshold.

A P0442 leak pulls down normally, then creeps back toward atmospheric pressure. That slow decay is the signature: the system can seal, just not tightly enough.

A large leak snaps back almost immediately and usually sets P0455. For this slower P0442 curve, inspect seals and hose ends first, then test the purge and vent valves, and use smoke for the pinhole you cannot see.

04:45Diagnostic order

So here is the order to actually test this, cheapest and most likely first.

Step 1. Is the correct fuel cap tight, with a clean, soft and undamaged seal? Remove it and inspect the rubber under bright light. Clean dirt from the seal and filler lip, check the neck for rust, then refit it fully. A seal can look present and still be too hard to hold the monitor's tiny pressure difference.

Step 2. Are the EVAP hoses and quick connectors intact at every bend and joint? Flex the short rubber sections gently and inspect the plastic lines where they clip to the body. Hairline splits hide on the underside of bends and connector O-rings flatten with age.

Step 3. Does the purge valve hold vacuum when commanded closed? Test it with the engine off using the factory procedure. It should seal when de-energised on most systems, but confirm the design first. Any steady bleed through the closed valve is large enough to fail P0442.

Step 4. Does the vent valve close fully and hold the test pressure? Command it closed with a capable scan tool and verify the system responds. Dust in the seat, a swollen seal or corrosion can leave a gap too small to see and large enough to fail.

Step 5. Does an approved low-pressure EVAP smoke test reveal the pinhole? Use purpose-built EVAP equipment, close the vent and follow the vehicle maker's pressure limit. Inspect the cap adapter, canister seams, pump-module seal and lines above the tank. Never pressurise a fuel system with oxygen or improvised shop air.

Cap and hose seals first, valve sealing second, smoke last. Small leaks reward patience and punish random parts replacement.

06:41Repair costs

What this costs to put right, in US dollars, parts and labor.

Fuel cap: 10 to 40 dollars in parts, plus about 0.1 hours of labor. Buy the correct cap; a universal cap can create the same leak.

EVAP hose, connector or seal: 10 to 70 dollars in parts, plus about 0.8 hours of labor. Cheap unless the failed joint is above the fuel tank.

Purge valve: 25 to 150 dollars in parts, plus about 0.7 hours of labor. Usually accessible in the engine bay and easy to bench-test.

Vent valve: 30 to 180 dollars in parts, plus about 1.0 hours of labor. Road dirt and corrosion make access and removal the variable.

Charcoal canister or tank-top seal: 100 to 500 dollars in parts, plus about 2.0 hours of labor. The expensive end and the reason smoke testing comes before parts.

Most P0442 repairs are a cap seal, hose end or valve under two hundred dollars. The costly canister and tank-top seals are exactly what should be proven with smoke before anything is ordered.

07:57Recap and related codes

Quick recap. P0442 means the fuel-vapour system sealed, then lost pressure slowly. Inspect the cap seal and every hose joint first, test the purge and vent valves for seepage, and use only approved low-pressure EVAP smoke equipment to find the pinhole that remains.

If your scan tool showed a related code alongside this one, check those too: P0455, P0456, P0446, P0440.

Codes are on screen and linked below.