P04AC code meaning
EVAP System Purge Control Valve B Circuit Low
Powertrain system Generic code P04xx codes Check engine light
What P04AC means
The evaporative emission (EVAP) system stores fuel vapour from the tank in a charcoal canister; a purge valve lets the engine draw it off, and a vent valve opens the canister to fresh air. Purge valve B is a second electric purge valve, usually for the high-load purge line of a turbo engine, which purges to the compressor inlet when there is boost in the manifold (Ford patent). Many makers solve this path with one purge valve and passive check valves instead – Ford tests it as P04F0 – and then have no valve B. CARB requires every purge path to be checked. P04AC means the control wire of purge valve B is shorted to ground: the valve stays energised and open, purging is uncontrolled and the mixture may run rich, especially at idle or after refuelling. No manufacturer document available to us uses this code; the description follows CARB and the comparable purge valve A checks (for GM P0443/P0458/P0459).
P04AC at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyEasy, DIY possible
- Typical labor0.3 to 1.5 hours
- Clears after repairAfter a drive cycle
Symptoms of P04AC
- Check engine light on
- Rough idle, rich mixture
- Starting problems after refuelling
Common causes of P04AC
- Most likelyControl wire of purge valve B chafed against ground
- PossibleCoil shorted internally
- PossibleMoisture in the connector
- Less likelyDriver in the engine module
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
EVAP purge valve bHigh load purge linePurge valve wiringEngine control module
What each part of P04AC means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 4 | Area: auxiliary emission controls (catalytic converter, EGR, EVAP). |
| AC | The exact fault in that area. |
What to do next
- Note P04AC and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P04AC
What does P04AC mean?
P04AC means "EVAP System Purge Control Valve B Circuit Low". The evaporative emission (EVAP) system stores fuel vapour from the tank in a charcoal canister; a purge valve lets the engine draw it off, and a vent valve opens the canister to fresh air. It is a generic code, so it means the same on every make.
Is it safe to drive with P04AC?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will P04AC fail an emissions test?
Yes, it can. P04AC is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P04AC to fix?
Difficulty: easy. A typical repair takes 0.3 to 1.5 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will P04AC clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.