Number P0442: EVAP System Leak Detected (small leak)
The evaporative emission (EVAP) system stores fuel vapour from the tank in a charcoal canister; a purge valve lets the engine draw it off, and a vent valve opens the canister to fresh air. P0442 reports a small leak; what size that means depends on the maker. Ford detects 0.040 in (1 mm) with the en
Can you drive with P0442?
Severity: Plan the repair. Yes — this is an emissions-system fault. The car will generally drive normally, but it will fail an emissions inspection with the light on.
If you ignore it: Fuel economy and emissions creep up, and some faults in this family (rich/lean running) stress the catalyst over time.
This is the #22 most common code in our top-150 library, in the evap — fuel vapor system family (Powertrain system). Severity is Number Nest's editorial triage — if the light is flashing or the car feels unsafe, stop driving it whatever any page says.
What P0442 means
The evaporative emission (EVAP) system stores fuel vapour from the tank in a charcoal canister; a purge valve lets the engine draw it off, and a vent valve opens the canister to fresh air. P0442 reports a small leak; what size that means depends on the maker. Ford detects 0.040 in (1 mm) with the engine running: the vent valve closes, engine vacuum pulls the tank down, the purge valve closes and a pressure rise above 2.5 inH2O in 15 s sets the code (0.020 in is P0456). GM uses the natural pressure change after switch-off for 0.020 in and averages several tests. Pump systems at VW and Audi compare against a reference leak. A worn or loose fuel cap is the most common cause.
Systems involved: evap system, fuel filler cap, evap hoses, fuel tank pressure sensor.
Causes, ranked by likelihood
| # | Cause | Likelihood |
|---|---|---|
| 1 | Fuel cap seal worn, wrong or not clicked in | most likely |
| 2 | Cracked or porous EVAP hoses or leaking connections | most likely |
| 3 | Purge or vent valve does not seal | possible |
| 4 | Leak detection pump itself leaking (VW/Audi) | possible |
| 5 | Filler neck corroded, tank or sender seal leaking | less common |
Check them in order — the list is ranked so the likely, cheaper causes come before the expensive part.
Symptoms you may notice
- Check engine light on
- Possibly slight fuel smell
- Readiness or emission inspection not passed
- No change in driving
Misdiagnosis traps
- Replacing the gas cap three times for a leak that is actually a cracked filler neck or a rusted vent line — a smoke test ends the guessing in one visit.
- Small-leak codes return after parts swaps when the real fault is a vent valve that seals on the bench but leaks under the car's own pressure test.
How a good shop confirms it
- Start with the free checks: cap gasket, filler neck rust, and any hoses disturbed by recent work.
- A smoke machine with the system sealed finds the leak by sight — insist on a located leak, not a parts list.
Repair options & costs
| Repair | Shop total (parts + labor) | DIY (parts only) | Difficulty |
|---|---|---|---|
| EVAP leak diagnosis (smoke test) + gas cap | $73 – $248 | $10 – $60 (parts only) | 1/5 |
| EVAP purge valve / solenoid | $68 – $198 | $30 – $110 (parts only) | 1/5 |
National ranges from Number Nest's cost model (midsize car, independent shop, US-average labor). Published cross-check — EVAP purge valve / solenoid: CarMD 2026 national average $172.
OBDex repair data for this code rates the job difficulty medium with a typical labor time in the 0.2–3 hour range and DIY possible: yes.
Price this repair in your state
Estimated total: — (midsize car, independent shop)
Frequently asked questions
Will P0442 fail an emissions test?
Yes in most states — an EVAP monitor fault fails the OBD portion even though tailpipe emissions are barely affected.
Is a smoke test worth paying for?
For small leaks, yes: it typically costs less than the second guessed part, and it converts "probably the valve" into a photographed leak.
How much does it cost to fix P0442?
The fix most associated with this code — evap leak diagnosis (smoke test) + gas cap — runs $73 – $248 at an independent shop for a midsize car in our cost model (parts + labor, US average). The code names a system, not a part, so confirm the cause before spending: the ranked causes above are ordered by likelihood.
Can I drive with P0442?
See the can-you-drive verdict above — it is based on the system involved and the cascading-damage risk, and it is our editorial triage, not a manufacturer instruction. If the light is flashing or the car feels unsafe, stop driving it regardless of any code.
Will P0442 clear itself?
If the fault was a one-off (a loose cap, a cold-start glitch), the light can go out after several clean trips and the code ages into history. A fault that is still present re-sets the code as soon as the car runs that self-test again — monitors run under specific conditions, which is why "it hasn't come back in two days" is not proof of a fix.
Will my car fail inspection with P0442?
In states with OBD-based emissions inspection, yes — an illuminated check-engine light or an incomplete readiness monitor fails the test regardless of what the tailpipe would show. C-codes (ABS) fall under safety inspection instead in states that have one.
What should I check first for P0442?
The first item on the ranked-causes list above — it is ranked likelihood-first for exactly this reason — plus the free checks: recent work in the area (connectors left loose are a classic), fluid levels, and the battery and grounds on electrical codes.
Where this data comes from
Code facts, causes and symptoms come from OBDex — an open dataset of independently authored descriptions of the standardized generic OBD-II codes (CC0 1.0, public domain; github.com/foerbsnavi/OBDex). The SAE J2012 document itself is copyrighted and is not reproduced here. Cost ranges are Number Nest's own US planning model: national parts and labor ranges cross-checked against the CarMD Vehicle Health Index 2026 published averages, with labor adjustable by the BLS OEWS May 2025 state wage index for auto technicians (occupation 49-3023) — a wage index, not a surveyed shop rate. Severity and can-you-drive notes are our editorial triage, not a manufacturer instruction.