Purge valve B mechanically stuck closed through dirt, carbon or internal wear; Electrical fault in the valve circuit such as an open circuit, high resistance, corroded connector or damaged wiring; Blocked, kinked or collapsed purge line or a blocked in-line check valve between the canister and intake (layout varies by engine); Valve coil failure so the valve does not respond to the control signal; Contaminated or saturated charcoal canister, often from repeated tank overfilling, restricting flow or fouling the valve; Fault in the control unit output driver for the valve.

P04AF

EVAP System Purge Control Valve B Stuck Closed.

Generic
Powertrain
Generic code
Low severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
4: AreaAuxiliary emission controls
AF: FaultThe specific fault in that area

What it means

P04AF means the engine control unit has detected that EVAP purge control valve B is not opening when commanded, so fuel vapour cannot flow from the charcoal canister into the intake. It usually sets when purge is commanded but the expected change in fuel trim, manifold pressure or EVAP system pressure does not follow. Which valve is 'B' and how it is monitored depends on the vehicle, so check the manufacturer's information for the model and engine.

Symptoms

Engine warning light on
Possible fuel smell around the car or fuel filler area
Difficulty filling the tank, with the pump clicking off early
Slightly rough idle or hesitation after refuelling
Other EVAP codes stored alongside it
Failed emissions test at inspection

Possible causes

Purge valve B mechanically stuck closed through dirt, carbon or internal wearlikely
Electrical fault in the valve circuit such as an open circuit, high resistance, corroded connector or damaged wiringlikely
Blocked, kinked or collapsed purge line or a blocked in-line check valve between the canister and intake (layout varies by engine)possible
Valve coil failure so the valve does not respond to the control signalpossible
Contaminated or saturated charcoal canister, often from repeated tank overfilling, restricting flow or fouling the valvepossible
Fault in the control unit output driver for the valverare

How to diagnose

Read all stored and pending codes and note the freeze frame data, including fuel trims, engine load and temperature at the time it set
Check for related EVAP, fuel trim or fuel level codes, and look for any relevant manufacturer service bulletins for the model
Inspect the valve connector, wiring and purge hoses for damage, corrosion, kinks, cracks and loose fittings
Use a scan tool's active test to command the valve and listen or feel for a click, and watch for a change in fuel trim or EVAP pressure
Check the valve supply voltage, control signal and ground at the connector, then measure the valve coil resistance against the manufacturer's specification
Test the valve and purge line for airflow and vacuum, with the valve both commanded and not commanded, to confirm it opens and closes properly
Carry out a smoke test of the EVAP system to find leaks and confirm the lines are clear, and check the canister for liquid fuel or debris
Check that any in-line check valves are fitted the right way round and pass flow in the correct direction

How to fix

Replace purge control valve B if it is stuck or its coil is faulty
Repair or replace damaged wiring, terminals or connectors in the valve circuit
Clear or replace blocked, kinked or collapsed purge lines and faulty check valves
Replace a contaminated or saturated canister and correct the cause, such as overfilling
Diagnose and repair or replace the control unit only after wiring and valve faults have been ruled out, following the manufacturer's procedure
Clear the code and road test, confirming the monitor completes and the code does not return

Can I keep driving?

Low severity

The car can normally be driven while you book it in, as this is an emissions fault and not a mechanical danger. Have it fixed soon because it may cause fuel odours, filling problems and an emissions test failure.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.