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P0440 and P0446: EVAP purge and vent valve faults
These two codes point at the valves that seal and breathe the fuel vapour system. A stuck purge valve is one of the few EVAP faults you can actually feel while driving.
P0440 “Evaporative Emission Control System Malfunction” and P0446 “Evaporative Emission System Vent Control Circuit” move the focus away from leaks and onto the hardware that controls the system. Unlike P0442, these are usually findable without a smoke machine.
The two valves and what they do
Purge valve (also called the purge solenoid). Sits between the charcoal canister and the intake manifold. Normally closed. The ECU opens it under specific conditions to draw stored fuel vapour into the engine to be burnt.
Vent valve (vent solenoid). Sits between the canister and fresh air. Normally open, letting the canister breathe. The ECU closes it to seal the system for leak testing.
Together they let the ECU perform its leak test: close the vent, open the purge, pull vacuum from the manifold, then close everything and watch whether the vacuum holds.
P0446: the vent side
The vent valve is nearly always mounted low on the vehicle, near the canister, which puts it in the firing line for water, salt and debris. Failures:
- Electrical: open coil, corroded connector, chafed wiring. This is what the code name refers to.
- Mechanically stuck open: the system can never be sealed, so leak tests fail and you also get leak codes.
- Mechanically stuck closed: this is the dangerous one. The tank cannot breathe. Vacuum builds as fuel is consumed, and you may hear a rush of air when opening the filler cap, or in extreme cases see the tank deform. It can also starve the fuel pump.
- Blocked filter: many vent valves have a small dust filter that clogs with road debris.
If opening the fuel cap produces a strong hiss or suction, treat the vent path as the primary suspect and do not ignore it.
P0440: the general one
P0440 is a broad code that different manufacturers use differently. It commonly indicates the system as a whole failed to behave as expected during a monitor run, which in practice usually means the purge valve.
Purge valve stuck open is the one fault in the whole EVAP family with real drivability symptoms. It becomes a permanent vacuum leak plus an uncontrolled dose of fuel vapour into the intake:
- Rough or hunting idle, especially just after refuelling — related: fluctuating idle RPM.
- Hard starting immediately after filling the tank, because a slug of vapour floods the intake.
- Lean or rich fuel trim swings that make no sense — see fuel trim explained.
- Sometimes P0171 at idle only.
That last pattern is a strong tell. A vacuum leak that only shows at idle and disappears under load, on a car that recently visited the petrol station, is a purge valve until proven otherwise.
Testing the valves
Bench test. Remove the valve, apply 12 V and listen. It should click firmly. Then blow through it in both states: a purge valve should be sealed unpowered and open when energised. A valve that passes air when unpowered is stuck open, and that is your fault found.
In-car actuator test. Mode 08 control operation or the manufacturer routine lets you command each valve and listen for the click without removing anything.
Live data. Watch purge duty cycle against manifold pressure and fuel trims. A commanded purge should produce a visible, brief disturbance in short-term fuel trim. No disturbance at all means no flow — a blocked line or a valve stuck shut.
Vacuum check. With the engine idling and the purge commanded closed, there should be no vacuum at the canister side of the valve. Vacuum present means it is leaking through.
The refuelling clue
Because both valves are involved in refuelling behaviour, symptom timing narrows things quickly:
| Symptom | Likely valve |
|---|---|
| Rough idle right after refuelling | Purge stuck open |
| Pump keeps clicking off during refuelling | Vent blocked or canister saturated |
| Hiss or suction when opening the cap | Vent stuck closed |
| Fuel smell around the rear of the car | Canister or hose, see fuel smell in cabin |
Cost expectations
- Purge valve: $20–120.
- Vent valve: $30–150.
- Vent filter: $15–50.
- Charcoal canister, if saturated by repeated overfilling: $100–400.
Overfilling the tank past the first click is a genuine cause of canister saturation. Liquid fuel in a charcoal canister destroys it.
After the repair
Clear the code, then complete a drive cycle with the tank between a quarter and three quarters full, and confirm the EVAP monitor reads complete. Background on why that monitor is so fussy: OBD drive cycle.
Try it on the free tier of the app — pair an ELM327 adapter and watch short-term fuel trim while purge is commanded. A trim that does not flinch when the valve opens tells you the vapour is not moving, whatever the valve sounds like.
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