OBD-II code · P0442
On this page
- What the code actually means
- Symptoms
- Is it safe to drive?
- What causes it — most common first
- How to diagnose it, in order
- 1. Tighten the fuel cap to 3+ clicks
- 2. Inspect the fuel cap rubber gasket
- 3. Visual inspection of evap hoses
- 4. DIY pressure test (no smoke machine)
- 5. Smoke test (definitive)
- 6. Test purge and vent valves
- Fixes, cheapest first
- How to reset the code after a repair
- What to do if it comes back
- Related OBD-II codes
P0442 Code: Evaporative Emission Control System Leak Detected (Small Leak)
What the code actually means
SAE J2012 defines P0442 as "Evaporative Emission Control System
Leak Detected (Small Leak)". The PCM seals the evap system during a
test cycle and watches for pressure to bleed off (or vacuum to bleed
in). If the leak rate matches a 0.020-inch orifice, this code sets.
The size hierarchy of evap leak codes:
P0440: Generic evap fault.P0442: Small leak (about 0.020 inch).P0455: Gross leak (about 0.040 inch and larger).P0456: Very small leak (about 0.010 inch on some platforms).
P0442 is the most common evap code on modern vehicles. It is a
two-trip code and only runs under narrow conditions: fuel level
1/4–3/4, ambient temperature 40–95 °F, and a specific drive pattern.
Symptoms
- Check engine light on (essentially the only symptom).
- Faint gasoline smell near the fuel filler, occasionally.
- Failed state emissions test until the code clears.
Is it safe to drive?
Yes, indefinitely. P0442 causes no drivability problems or engine
damage. The escaped fuel vapor is an emissions violation but does not
affect how the car runs.
What causes it — most common first
1. Worn or loose fuel cap (~40%). Modern caps have rubber gaskets that age, crack, and stop sealing tightly. Clue: cap doesn't click 3 times; gasket has visible cracks or hardening.
2. Cracked or aged evap hose (~40%). Rubber and plastic vapor lines between the tank, canister, and engine intake develop hairline cracks. Clue: visible cracking or stiffness in the hoses; smoke test shows leak point.
3. Failed purge valve (~10%). Valve diaphragm or seal leaks vapor past the closed position. Clue: commanded closed but still allows air flow on bench test.
4. Failed vent valve / CCV (~5%). Stuck partly open. Clue: PCM can't fully seal the system during self-test.
5. Failed fuel-tank pressure sensor (~3%). Reports phantom small leaks. Clue: sensor voltage stuck at one value across conditions.
6. Loose canister-purge hose at the intake (~2%). Connection at the intake manifold worked loose. Clue: visible gap or hiss at idle.
How to diagnose it, in order
1. Tighten the fuel cap to 3+ clicks
Free first step. Clear the code with a scan tool (or wait for it to
self-clear) and drive 200+ miles through varied conditions. About 40%
of P0442 codes never return after this step.
2. Inspect the fuel cap rubber gasket
Pull the cap. Look at the rubber O-ring on the underside. Cracks, hardening, or a flattened ridge mean the seal is done. Replace with an OEM cap for about $15–$30.
3. Visual inspection of evap hoses
Follow every visible vapor hose from the filler back to the canister and forward to the purge valve. Look for:
- Cracked rubber at clamps.
- Disconnected hoses.
- Hardened plastic lines that no longer flex.
- Rodent damage.
About 30% of the codes that survive the cap fix resolve here.
4. DIY pressure test (no smoke machine)
A low-cost option: connect a low-pressure nitrogen or compressed-air source (regulated to under 1 psi — anything more risks splitting hoses) to the service port. Spray soapy water on every joint and listen for hissing. Bubbles or hissing isolate the leak.
If you don't have a regulated nitrogen source, your local shop's smoke test ($60–$120) is the next step.
5. Smoke test (definitive)
A smoke machine introduces low-pressure mineral-oil smoke. Smoke escaping anywhere shows the leak. This catches 90% of leaks that visual inspection misses.
6. Test purge and vent valves
Command each valve open/closed with a scan tool. A working purge valve clicks audibly and seals on command.
Fixes, cheapest first
| Fix | Cost (USD) | When it applies |
|---|---|---|
| Tighten the existing cap to 3+ clicks | $0 | Code is recent; never tried |
| Replace the fuel cap | $15–$30 | Gasket cracked or cap fails to click |
| Replace a cracked evap hose | $5–$50 | Visible damage; smoke test confirmed |
| Replace purge valve | $20–$120 | Bench test fails |
| Replace vent valve / CCV | $30–$150 | PCM can't seal during self-test |
| Replace charcoal canister | $80–$300 | Physical damage |
| Smoke test service | $60–$120 | After visual inspection finds nothing |
The cap-first approach saves diagnostic time and is what most independent shops try before quoting smoke service.
How to reset the code after a repair
Clear with a scan tool, then complete 2–3 full drive cycles matching the evap monitor conditions (1/4 to 3/4 fuel level, mild ambient temperature, mixed driving, 8+ hours engine-off between cycles). The evap monitor is one of the slowest readiness flags to re-set.
Without a scan tool, expect 200–400 miles before the readiness flag sets.
What to do if it comes back
- Within a week of cap replacement: the leak was downstream of the cap. Smoke test next.
- After replacing a hose: check the hose you replaced for proper seating; small leaks often hide at hose clamps.
- Returns after every fix: fuel tank pinhole or filler neck rust — common on vehicles over 15 years old in rust-belt regions.