Emissions and evaporative
P0442Small evaporative emission system leak detected
EVAP System Leak Detected (small leak)
Get it diagnosed soon
Generic OBD-II code
Evaporative Emission (EVAP) System Component
What this part does
Captures and purges fuel vapor from the fuel tank/charcoal canister into the engine rather than releasing it to the atmosphere; includes the purge and vent solenoids and related sensors.
What the computer is measuring
Petrol evaporates, and the vapour is a pollutant. The evaporative emission system exists to catch it. Vapour from the fuel tank is routed to a canister of charcoal that holds it, and when the engine is running under the right conditions a purge valve opens and draws that stored vapour into the intake to be burned.
For that to work the whole system has to be sealed. So the computer tests it. Depending on the vehicle it either draws a vacuum on the system and watches whether it holds, or it seals the system and watches for the natural pressure change as the fuel cools. Either way it is looking for the system to hold what it is given.
P0442 is the result when the leak it detects is a small one. Manufacturers set that threshold at a hole of roughly half a millimetre, which is smaller than most people picture. You will not hear it, you will usually not smell it, and there is nothing wrong with how the car drives. That is why this code is almost always found by the light rather than by a symptom.
The order of the checklist matters here more than on most codes. The fuel cap is genuinely the most common cause, it takes a minute to check, and a great many of these are solved without opening anything. Work through the free checks before anyone charges you for a smoke test.
What you will notice
- Steady check engine light and nothing else. The car drives exactly as normal.
- Occasionally a faint fuel smell near the rear of the vehicle.
- The code often appears after refuelling and may clear itself for a while.
Likely causes, most common first
Ordered by how often each one actually turns out to be the problem, not alphabetically and not by part price.
A loose, cracked or worn fuel filler cap
The single most common cause and the cheapest fix. The seal hardens with age, so a cap that clicks can still leak.
A cracked or disconnected vapour hose
Rubber lines between the tank, the charcoal canister and the engine harden and split, usually where they bend or where road salt reaches them.
A leaking purge valve or vent valve
The two solenoids that seal the system. A valve that does not close fully leaks continuously and the computer cannot tell it apart from a hose leak without testing.
A failed seal at the charcoal canister or the fuel tank sending unit
More common on older vehicles and on anything that has had the fuel pump replaced, because the tank seal is disturbed during that job.
A pinhole in the fuel tank or filler neck
Rust at the filler neck is common on vehicles from salted road regions and is the failure that actually smells of fuel.
What to check, in order
Remove the fuel cap, inspect the rubber seal for cracks and hardening, refit it until it clicks, then clear the code and drive for a few days.
If the code does not return, the cap was the leak. This is worth doing first every time because it costs nothing.
Follow the vapour hoses from the fuel tank forward to the engine, looking for splits, chafing and disconnected fittings.
A visible split explains the code. Intact hoses move the search to the valves and the tank.
Inspect the filler neck where it meets the tank, particularly on a vehicle that has seen winter salt.
Visible rust perforation is the leak and no further testing is needed.
Have the system smoke tested if nothing is visible.
A small leak by definition is one you cannot usually see or hear. Smoke is the only reliable way to locate it, and it either finds the leak or proves the system is sealed.
Where this one turns up most
- Vehicles over about eight years old, where the filler cap seal has hardened.
- Anything driven on salted winter roads, where the filler neck rusts through.
Codes that often show up alongside it
What a DriveDecode adapter can read
A DriveDecode adapter reads engine and emissions systems: the powertrain and emissions codes the OBD-II standard requires on every US vehicle it covers. It does not read ABS, airbag (SRS), TPMS or transmission-specific manufacturer codes. Those need make-specific decoding that generic OBD-II hardware cannot do.
Reading P0442 on your own car
DriveDecode pulls P0442 straight off the vehicle along with the freeze frame the computer stored when it set: the coolant temperature, engine load and speed at that exact moment. That context is what separates a fault that happens on a cold start from one that happens under load, and it is the first thing the list above asks you to narrow down.
Last reviewed 2026-09-05. Definition sourced from the published standard.