What does it mean?
P04B1 tells you the control circuit for the refueling vapor control valve is being pulled to a voltage below the module's low threshold. The module drives the valve and monitors its own output line; when that line reads near ground at a point in the cycle where it should be at supply level, the module logs a low circuit fault. The usual cause is an unintended path to ground in the harness or inside the valve, and the practical consequence is that the module loses authority over the valve and shuts the driver down to protect it. Because this valve is only used at refuelling and during evaporative tests, the customer often notices nothing until the tank needs filling. On systems with a controlled fuel filler the fault can block refuelling entirely.
Symptoms
- MIL (check engine lamp) on with evaporative codes stored
- Refuelling blocked or delayed on vehicles where the module controls the filler
- Fuel nozzle shutting off repeatedly during filling
- Fuel odour near the rear of the vehicle
- The valve does not respond correctly to scan tool commands
- Evaporative readiness monitor that never completes
Likely causes
- Control wire chafed to the underbody, a heat shield or a fuel tank strap, shorting to ground
- Water and salt intrusion into the valve connector bridging the control pin to ground
- Internally shorted solenoid winding measuring well below its specified resistance
- Harness pinched during tank removal, exhaust work or rear suspension service
- Damaged loom where the evaporative wiring crosses a sharp bracket edge under the vehicle
- Module driver stage shorted internally and holding the output low
How to fix it
- Determine whether the fault is present now, then disconnect the valve and clear the code to see whether the fault changes to an open circuit report
- With the key off and the valve unplugged, measure resistance between the harness control pin and ground; a very low value confirms a wiring short
- Measure the valve winding resistance and compare against specification to identify an internally shorted solenoid
- Inspect the underbody harness run for chafing against the tank straps, brackets, heat shields and the exhaust
- Open the connector and check for water, corrosion, damaged seals and green terminals, drying and treating as needed
- Repair the shorted wire properly and re secure the loom so it cannot rub in the same spot again
- Clear the code, actuate the valve, then perform a refuelling event to confirm the system releases and operates normally