P0442 means the evaporative emission (EVAP) monitor detected a small leak in the fuel vapor system. "Small" has a definition: the monitor is calibrated to catch a leak equivalent to a hole about 1 mm (0.040 in) across. Its neighbors are P0455 (large leak, about 1 mm and up on older calibrations or a gross leak such as a missing cap) and P0456 (very small leak, about 0.5 mm). The system being tested runs from the fuel tank through the charcoal canister and purge valve to the intake, and every hose, seal and valve in that path is a candidate.
Severity: low. Can you keep driving? Yes. The leak is fuel vapor, not liquid fuel, and the engine runs normally. The consequences are an emissions test failure and, on many states' inspections, a rejection for the check engine light. Nothing is damaged by waiting, but the light will not clear on its own until the leak is fixed and the monitor has run clean.
How the monitor finds a pinhole
The engine module seals the system by closing the canister vent valve, then either pulls a vacuum through the purge valve (most designs) or pumps it up with a leak detection pump (Chrysler, Audi and Volkswagen designs). It then watches a tank pressure sensor. If the vacuum decays or the pressure falls faster than a calibrated rate, a leak is declared. The test only runs when fuel level, ambient temperature, coolant temperature and drive time are all inside a window, which is why the code can take days to set or return.
That mechanism produces the single most useful diagnostic tip on this code, from General Motors bulletin 21-NA-124 (December 2022, all GM gasoline vehicles 2000 to 2023): during the P0442 test the vent valve is under vacuum, but a shop smoke or pressure tester pressurizes the system to about half a psi. A vent valve that seals when pressurized can leak under vacuum. A system that passes a pressure smoke test and still sets P0442 has a valve that behaves differently in the two directions.
The gas cap, honestly
Nissan's bulletin NTB13-097g (January 2019, all 2012 and newer Nissans except LEAF and the Titan XD diesel) says a loose filler cap left loose for several trips stores P0442, P0455 or P0456 and turns the light on. It also says something most gas cap advice leaves out: do not test-drive to confirm the code; use the diagnostic flow chart, because the monitor's conditions are too narrow for a drive to prove anything. So: check the cap first, tighten it until it clicks, look at its seal for cracks, and then, instead of driving for a week to see, run the EVAP leak test with a scan tool that can command the system sealed.
Causes, in the order they are found
Filler cap with a cracked or hardened seal, or a non-original cap that does not fit the neck.
Filler neck and its seals. GM bulletin PI1034 (August 2013) traced P0442 on the 2014 Chevrolet Impala 3.6 L to a filler pipe housing seal that was not seated at assembly and overlapped the neck; the fix was to push the seal into place with a trim stick, 0.4 hour. Volkswagen's tech tip for the 2018 to 2019 Atlas points at the connections behind the rubber boot in the filler door, the canister breather line and the filler neck collar, as a common leak found with soapy water.
Vent valve or purge valve that leaks or sticks partly open. GM's tip: check the vent valve under vacuum, not just pressure.
Hoses and lines cracked by age, rubbed through near the tank straps, or disconnected after a fuel pump or tank job.
Charcoal canister cracked, or saturated with liquid fuel from repeated overfilling (shake it; VW's tip says listen for a rattle).
Leak detection pump (Chrysler, Audi, VW) with its own diaphragm or vacuum switch leaking, which Audi's Q7 bulletin lists as a cause on its own.
Fuel tank seam or rollover valve, least common.
A diagnostic order
- Record the freeze frame and note companion codes. P0441 or P0496 (purge flow) beside P0442 points at the purge valve rather than a hose.
- Cap first, and check the neck seal by hand.
- Command the system sealed with a scan tool and watch tank pressure to confirm the leak is present now.
- Smoke-test, but test the vent valve under vacuum as well as under pressure.
- Soapy water at the filler neck connections if the smoke test is inconclusive.
- Fix, clear, and run the EVAP monitor with the scan tool's drive cycle rather than waiting; the readiness monitors need it to complete before an inspection anyway.
The full family, including what P0455 means beside this code, is covered in EVAP codes P0442 and P0455 and the fuel cap.
Frequently asked questions
Can I drive with a P0442 code?
Yes. The leak is vapor, the engine runs normally, and nothing is damaged by waiting. The costs are an emissions test failure and a light that stays on.
Will tightening the gas cap fix P0442?
If the cap was the leak, yes, but the light will not go out until the monitor has run clean, which can take several drives. Nissan's own guidance is to confirm with the scan tool's EVAP test rather than by driving.
Why did the smoke test pass but the code came back?
Because the shop test pressurizes the system and the car's monitor pulls a vacuum. GM's bulletin notes that a canister vent valve can seal under pressure and leak under vacuum. Test the vent valve in the direction the car tests it.
How much does it cost to fix P0442?
A cap or a hose is inexpensive. A purge or vent valve is a modest part with a short replacement. A filler neck, canister or leak detection pump is more. The diagnostic time is usually the larger cost, which is why the scan tool test comes before the smoke machine.
Related codes
- P0455: large leak. P0456: very small leak.
- P0441 / P0496: purge flow incorrect or high.
- P0446 / P0449: vent control circuit.
- P0452 / P0453: fuel tank pressure sensor low or high.
The code number itself tells you the family and the domain; how to read an OBD2 code number explains what each character means.