What does it mean?
P04AC reports that the control circuit of the second evaporative purge control valve is being held at a lower voltage than the module expects. The module drives the valve with a pulsed ground and monitors its own output; when that output reads near zero at a time when it should be high, the only explanations are an unintended path to ground or a driver that is stuck on. A short to ground can make the valve stay energised regardless of what the module wants, which means vapour can flow into the intake at times when the engine is not ready for it. To protect the driver stage and the engine, the module usually disables the output and stores this code. Fuel trim disturbance and a rough idle are common side effects while the fault is active.
Symptoms
- MIL (check engine lamp) on, often with fuel trim or idle quality codes alongside
- Rough or hunting idle, especially right after a hot restart
- Hesitation or a stumble shortly after starting while unmetered vapour enters the intake
- Difficult starting after refuelling on some applications
- Fuel odour and an evaporative monitor that never completes
- No control over the valve when it is commanded with a scan tool
Likely causes
- Control wire chafed through to the engine block, a bracket or a metal clip, creating a short to ground
- Moisture or corrosion inside the purge valve connector bridging the control pin to ground
- Internally shorted solenoid winding with resistance far below specification
- Harness pinched during previous intake, cover or battery tray removal
- Damaged loom where the evaporative harness passes near the exhaust or a moving component
- Module driver stage shorted internally and holding the output low
How to fix it
- Check whether the fault is currently present, then unplug the purge valve and clear the code to see if the fault changes character
- With the valve disconnected and the key off, measure resistance from the control pin of the harness to ground; a very low value proves a wiring short
- Measure the solenoid winding resistance and compare against specification to rule out an internally shorted valve
- Inspect the harness from the module to the valve for chafe points, especially at brackets, clips and any place the loom crosses metal
- Open and dry the connector, checking for water intrusion, corrosion and damaged seals
- Repair the shorted section with correct gauge wire and proper insulation rather than patching it temporarily
- Clear the code, command the valve through its range and confirm the purge duty cycle now responds normally with stable fuel trims