Damaged or disconnected wiring harness to the purge control valve B; Corroded or loose electrical connector at the valve; Failed purge control valve B solenoid (open or shorted coil); Short to ground in the control circuit between the PCM and the valve; Faulty powertrain control module driver circuit (less common); Blown fuse or poor ground connection shared with the EVAP circuit

DTC codes / P04xx · Auxiliary emission controls / P04AC

P04ACEVAP System Purge Control Valve B Circuit Low

Q: What does code P04AC mean?

EVAP System Purge Control Valve B Circuit Low. P04AC means the powertrain control module (PCM) has detected a low-voltage condition on the electrical circuit for the EVAP system's purge control valve B, which is the valve that meters fuel vapor from the charcoal canister into the intake. The PCM monitors the valve's control circuit and sets this code when the signal voltage stays below the expected threshold, indicating a short to ground, an open circuit, or a faulty valve.

P04AC means the powertrain control module (PCM) has detected a low-voltage condition on the electrical circuit for the EVAP system's purge control valve B, which is the valve that meters fuel vapor from the charcoal canister into the intake. The PCM monitors the valve's control circuit and sets this code when the signal voltage stays below the expected threshold, indicating a short to ground, an open circuit, or a faulty valve.

SystemPowertrain

Code typeGeneric (all makes)

SeverityLow

Fix difficultyIntermediate

AreaAuxiliary emission controls

Symptoms you may notice

Illuminated check engine light, often with no other symptoms
A faint fuel or gasoline odor around the vehicle, especially after refueling
Slightly rough idle or a stumble when the purge valve is commanded open
Failed emissions inspection due to the check engine light
In rare cases, a noticeable drop in fuel economy if the valve sticks open
No drivability change at all in many instances

Common causes, most likely first

Damaged or disconnected wiring harness to the purge control valve B
Corroded or loose electrical connector at the valve
Failed purge control valve B solenoid (open or shorted coil)
Short to ground in the control circuit between the PCM and the valve
Faulty powertrain control module driver circuit (less common)
Blown fuse or poor ground connection shared with the EVAP circuit

How to diagnose and fix it

Scan all modules and record freeze-frame data to confirm the code and check for related EVAP codes
Inspect the purge valve B connector and wiring for corrosion, chafing, or loose pins
Measure resistance across the valve solenoid and compare to the manufacturer's specification
Check for battery voltage at the valve connector with the key on and verify the ground circuit
Test the control circuit for a short to ground or open using a wiring diagram and multimeter
Replace the purge control valve B if electrically faulty, then clear codes and verify the repair
If the circuit tests good but the code persists, diagnose the PCM driver circuit before replacing the PCM

What a low purge valve circuit really means

A low-voltage code on a purge valve circuit is most often a wiring or connector problem rather than a failed valve, so inspect the harness before condemning the part.
The PCM may set P04AC intermittently if the valve coil resistance drifts with temperature, so a cold resistance check can pass while a hot engine triggers the code.
A stuck-open purge valve can cause a rough idle and fuel smell that mimics a vacuum leak, yet the code points to an electrical low, not a mechanical failure.
Some vehicles use a shared ground for multiple EVAP components, so a single corroded ground point can set P04AC along with other purge or vent codes.
Clearing the code without repairing the circuit will usually result in the code returning after one or two drive cycles once the monitor runs.

Can I keep driving with P04AC?

The vehicle is generally safe to drive with P04AC because the fault affects emissions control rather than engine operation, but avoid ignoring a persistent fuel smell and get the circuit inspected soon to prevent a failed emissions test.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.