P04AC: EVAP System Purge Control Valve B Circuit Low

What does P04AC mean?

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).

The code tells you which monitor failed, not which part is broken. Test before replacing parts.

Symptoms

• Check engine light on
• Rough idle, rich mixture
• Starting problems after refuelling

Common causes

• High Control wire of purge valve B chafed against ground
• Medium Coil shorted internally
• Medium Moisture in the connector
• Low Driver in the engine module

Likelihood is a general estimate across vehicles; your make and model may differ.