P0442
- Powertrain
- Generic (SAE)
- Emissions
Not urgent
Vapor containment only. Nothing mechanical is at risk and driveability is normally unaffected, but the MIL stays on and the vehicle will fail an OBD emissions inspection. A strong raw fuel smell or any visible liquid fuel is a separate and urgent matter.
- Safe to drive
- Safe to drive. Check the fuel cap first. Strong raw fuel smell or visible liquid fuel is a different problem — get that inspected immediately.
Evaporative Emission System Leak Detected (Small Leak)
The fuel vapor system leaked slowly during its self-test — a leak roughly the size of a 0.040 inch hole. Too small to see, big enough for the computer to measure.
The PCM seals the EVAP system, applies vacuum or pressure, and measures the decay rate against a calibration derived from a reference orifice, typically 0.040 inch (1.0 mm). A decay rate matching that orifice sets P0442. A smaller leak, around 0.020 inch, sets P0456 on most vehicles; a gross leak sets P0455. Because the criterion is a decay rate rather than a simple pass or fail seal, the result is sensitive to fuel level, temperature swing, and how full the tank was when the monitor ran.
How the car detected itNon-continuous monitor with tight enable conditions: fuel level usually between roughly a quarter and three quarters, ambient temperature within a moderate window, and either a steady drive or a long key-off soak on systems that use the natural vacuum created as fuel cools. The PCM closes the vent valve, draws vacuum through the purge valve or runs a leak-detection pump, then watches the fuel tank pressure sensor. It normally requires two consecutive failed runs before the MIL lights, which is why P0442 often appears as a pending code first, and why it can take a week to reappear after an ineffective repair.
A trouble code records what a control module measured. It does not identify which part failed. Test before replacing anything.
- Safe to drive
- Safe to drive. Check the fuel cap first. Strong raw fuel smell or visible liquid fuel is a different problem — get that inspected immediately.
- Urgency
- Not urgent
- Will it pass emissions
- No
- If ignored
- The light stays on and hides any new code behind it, and the vehicle fails emissions testing. Vapor escapes rather than being burned. One of the possible causes — a purge valve leaking through when closed — can also cause rough idle or hard starting after refueling, which does affect how the car drives.
- Check engine light
- Solid
- Clearing the light
- A completed drive cycle is normally required before the monitor re-runs and the light can clear.
- Possible smell of fuel near tank or engine bay
- Failed emissions inspection
- Loose-cap warning may appear
Many vehicles show no symptom at all beyond the warning light. The absence of a symptom does not mean the fault is not real.
These are the causes this code can have. For the order to work through them in, see what to check first.
| # | Cause | How often | Component |
|---|---|---|---|
| 1 | Fuel cap loose, missing, or worn seal | Very common | — |
| 2 | Small crack in EVAP hose | Common | — |
| 3 | Purge valve leaking | Occasional | — |
Ordering reflects how often each cause is responsible in general, not a probability for your vehicle. Confirm by testing.
In order. Each of these is cheaper or faster than what follows it, and each one can make the rest unnecessary.
The fuel cap seated with an audible click, and the condition of its seal
A common cause and free to check, so it goes first on cost alone rather than on odds. Remove it, inspect the O-ring for hardening or cracking, wipe the filler neck sealing surface, and refit until it clicks.
Whether the code is pending or confirmed, and how many drive cycles it has failed
A single pending P0442 after refueling may simply be a cap that was not tightened once. A confirmed, repeating code is a real leak worth chasing.
Whether the tank is habitually filled past the pump's first click
Topping off pushes liquid fuel into the charcoal canister. That damages the canister, produces EVAP leak codes, and causes hard starting after fuel stops.
Purge valve leak-through
A purge valve that bleeds slightly when it should be closed lets vacuum escape during the test and reads as a small leak, with no crack anywhere in the system.
Recent work or impact damage near the fuel tank, canister, or filler neck
A quick-connect not fully seated after fuel pump or tank work is a very common small-leak source and easy to miss.
Confirm the code status and read freeze frame
Tools Scan tool with freeze frame and readiness monitor status
Note whether it is pending or confirmed and record fuel level, ambient temperature and engine conditions at the failed run. This confirms the monitor actually ran within a valid window rather than producing a marginal result.
ExpectedConfirmed code with plausible enable conditions in freeze frame. A pending code with the tank nearly full or nearly empty may not be trustworthy.
Inspect and test the fuel cap and filler neck
Tools Known-good cap, light
Fit a known-good cap if one is available. Inspect the neck for rust, dents, and debris on the sealing surface. On a capless system, inspect the flapper assembly instead of looking for a cap.
ExpectedCap sealing cleanly on a smooth, clean neck surface, or an intact flapper on capless systems.
Smoke test the sealed system
Tools EVAP smoke machine, bidirectional scan tool, UV light and dye where the machine supports it
This is the reliable way to find a 0.040 inch leak. Command the vent valve closed, introduce low-pressure smoke at the service port, and inspect the whole path: filler neck, tank seams, fuel pump module flange O-ring, canister body and ports, vent and purge lines, and every quick-connect. Use an EVAP-rated machine only — shop-air pressure damages the system.
ExpectedNo smoke anywhere. When smoke appears, it is most often at a hose joint, the canister, the fuel pump module seal, or the filler neck.
Test the purge valve for leak-through
Tools Hand vacuum pump, bidirectional scan tool
With the valve unpowered and therefore closed, apply vacuum or low-pressure air to the engine side and confirm it holds. A purge valve bleeding when closed is a frequent small-leak cause and is overlooked constantly because nothing looks wrong.
ExpectedValve holds vacuum with no bleed-down when closed, and opens fully when commanded open.
Test the vent valve and leak-detection pump where fitted
Tools Bidirectional scan tool, hand vacuum pump
Confirm the vent valve seals when commanded closed and releases when commanded open. On systems with a leak-detection pump, run the pump's self-test and read its current-draw or pressure trace, which reports leak size directly on some vehicles.
ExpectedVent valve seals and releases on command; the pump builds and holds its reference pressure.
Check the fuel pump module seal and tank-top connections
Tools EVAP smoke machine, service information for access
The large O-ring where the fuel pump module sits in the tank is a common slow leak, especially after a pump replacement. Access varies enormously by vehicle — under a rear seat or cargo floor on some, requiring tank removal on others. If the module has to come out, relieve fuel pressure, disconnect the battery, and work away from ignition sources: an open tank vents raw vapor.
ExpectedNo smoke escaping at the module flange or the tank-top line connections.
Repair, then deliberately drive the enable conditions
Tools Scan tool with readiness monitor status
Keep the tank between roughly a quarter and three quarters, drive normally, and allow several complete drive cycles including overnight cool-downs. Only an EVAP monitor showing complete and passing confirms the repair.
ExpectedEVAP readiness monitor complete and passing, with no return of the code.
| Repair | Only after you confirm | Typical cost | Who |
|---|---|---|---|
| Replace the fuel filler cap | Worn or cracked cap seal, or smoke escaping at the cap during the test | $20–$70 | DIY |
| Replace the EVAP purge valve | Valve fails to hold vacuum when closed, or leaks through on a bench test | $130–$400 | DIY |
| Replace a cracked EVAP hose or reseat a quick-connect | Smoke escaping at that specific point during the test | $100–$400 | DIY |
| Replace the fuel pump module seal or tank O-ring | Smoke escaping at the module flange with the system pressurized | $200–$700 | Hard |
| Replace the EVAP vent valve | Valve fails to seal fully when commanded closed | $150–$450 | DIY |
| Replace the charcoal canister | Smoke escaping from the canister body, or the canister saturated with liquid fuel | $250–$900 | Hard |
| Repair or replace the filler neck or capless flapper assembly | Smoke escaping at the neck with a known-good cap fitted | $250–$800 | Hard |
Start with a $20 cap. If that is not it, budget $100 to $180 for a smoke test — hunting a 0.040 inch leak without one wastes more money in guessed parts than the test costs. Most confirmed repairs land between $150 and $500. Tank-mounted work sits at the expensive end almost entirely because of access labor, not part price.
- Replacing the cap, clearing the code, and heading straight to an emissions test. Clearing resets the readiness monitors and the test then fails for incomplete monitors.
- Calling the repair good because the light has not come back after two days. The EVAP monitor may not have run at all yet.
- Skipping the purge valve because nothing is visibly wrong with it. Leak-through is silent and invisible.
- Using shop air pressure instead of an EVAP-rated smoke machine. It damages canisters and valves.
- Testing with a nearly full or nearly empty tank, when the monitor will not run in the first place.
- Replacing the charcoal canister on suspicion. It is a genuine cause but not a first move.
- Fuel Filler Cap
- EVAP Canister
- EVAP Purge Valve
- Definition
- Generic code set (SAE J2012 / ISO 15031-6) via the OBDexopen database, dedicated to the public domain under CC0-1.0.
- Diagnostic guidance
- Written and reviewed in-house. See our editorial policy.
- Review status
- Reviewed
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