Faulty purge control valve B with internal short or low coil resistance; Wiring short to ground in the valve control circuit; Damaged, corroded or loose connector at the valve or control unit; Chafed harness near the valve, fuel tank or engine bay causing low circuit voltage; Missing or faulty power supply or fuse to the valve circuit; Fault inside the engine control unit driver circuit.

P04AC

EVAP System Purge Control Valve B Circuit Low.

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
AC: FaultThe specific fault in that area

What it means

P04AC means the engine control unit has detected a low voltage or signal in the control circuit of the second evaporative emission purge control valve, referred to as valve B. The control unit commands the valve and monitors the circuit, and it saw a lower voltage than expected, which suggests a short to ground or an open or faulty driver circuit. The number and layout of purge valves vary by model and engine.

Symptoms

Engine warning light on
Fuel smell around the car or fuel filler area
Rough idle or occasional stalling after refuelling
Difficulty starting after refuelling
Poor fuel economy in some cases
Possible additional EVAP codes stored

Possible causes

Faulty purge control valve B with internal short or low coil resistancelikely
Wiring short to ground in the valve control circuitlikely
Damaged, corroded or loose connector at the valve or control unitlikely
Chafed harness near the valve, fuel tank or engine bay causing low circuit voltagepossible
Missing or faulty power supply or fuse to the valve circuitpossible
Fault inside the engine control unit driver circuitrare

How to diagnose

Read all stored codes and freeze frame data to see engine conditions when the fault set, and note any other EVAP codes
Check the manufacturer's information to locate purge valve B on this model and engine, as layout varies
Visually inspect the valve, its hoses, harness and connector for damage, corrosion, fuel contamination or water ingress
Using a scan tool, command the valve on and off and listen for a click or feel for operation
Measure the valve coil resistance and compare it with the specification for the vehicle
Check the supply voltage and fuse at the valve connector with the ignition on
Test the control wire for a short to ground and for continuity back to the control unit with the connector unplugged
Clear the code and road test to see if it returns, only after the repair

How to fix

Replace purge control valve B if its resistance or operation is out of specification
Repair or replace the shorted or chafed wiring in the control circuit
Clean, repair or replace the damaged connector terminals
Restore the missing power supply by correcting the fuse or supply wiring fault
Replace or repair the engine control unit only after the wiring and valve are proven good and the manufacturer's procedure confirms it

Can I keep driving?

Low severity

The car can usually be driven normally in the short term, but book it in soon as a stuck-open valve could cause fuel vapour or running problems. It will not pass an emissions check while the fault is present.

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