P0499: EVAP Vent Valve Control Circuit High
P0499 means the engine computer found the control wire of the EVAP vent valve shorted to positive voltage, so the valve cannot close and the fuel vapor system cannot be sealed for testing. It is a low-urgency emissions fault that turns on the check engine light but usually does not change how the car drives.
You can keep driving
The code does not cause limp mode and driving usually feels normal. The EVAP system stays unsealed, so the car may release fuel vapor and cannot pass an emissions readiness check until it is repaired.
What it means
The evaporative emission (EVAP) system traps fuel vapor from the tank in a charcoal canister instead of letting it escape into the air. A purge valve lets the engine draw that vapor off, and a vent valve opens the canister to fresh air. The vent valve is normally open, and the engine computer closes it by switching its control wire to ground. P0499 sets when that wire stays at high voltage even while the computer is commanding the valve on. The data gives example thresholds: GM treats 0.5 ohms or less to the supply as this fault, and Saab looks for voltage above 4.6 V. The check engine light comes on after two trips with the fault. Because the valve cannot close, the computer suspends the leak tests, since they would otherwise report a large leak that is not really there.
Likely causes
- Control wire touching battery positive or the valve supply (chafed harness)Very likely
A worn wire insulation lets the control wire contact a powered wire, which holds it at high voltage.
- Vent valve coil with shorted turns, driver switches offPossible
A damaged coil can draw too much current, so the computer's driver shuts off and the wire reads high.
- Moisture or corrosion in the connectorPossible
Water or corrosion can bridge pins in the connector and put supply voltage on the control wire.
- Driver in the engine moduleLess likely
The internal switch that grounds the valve can fail, though this is the least common cause.
What you may notice
- Check engine light on
- EVAP readiness monitor not set
- Usually no change in how the car drives
What to check, cheapest first
- 1Read the code and any related codes such as P0449 or P0498, then note whether they are stored together. · you can do it
- 2Inspect the vent valve connector and its wiring for moisture, green or white corrosion, bent pins and chafed insulation. · you can do it
- 3Unplug the vent valve, clear the code, and see whether it returns, which helps separate a wiring fault from a valve fault. · you can do it
- 4Check the vent valve coil resistance against the specification for the vehicle to find shorted turns. · mechanic
- 5Test the control wire for a short to battery positive or the valve supply and trace along the harness to find the chafed spot. · mechanic
- 6If the wiring and valve test good, check the driver output in the engine module before replacing it. · mechanic
Related codes
Frequently asked questions
Can I drive with a P0499 code?+
Yes. The code normally does not affect drivability, but fix it soon because the EVAP system cannot seal and the check engine light stays on.
Can a loose gas cap cause P0499?+
Unlikely. P0499 is an electrical fault in the vent valve control circuit, not a leak. A loose cap typically sets leak codes such as P0455.
Will P0499 pass an emissions test?+
Probably not. The check engine light is on and the EVAP readiness monitor will not complete until the fault is repaired and a drive cycle is finished.
How hard is the repair?+
It is rated easy and doable at home in many cases, with an estimated 0.3 to 1.5 hours, depending on whether it is a connector, wire or valve.
Written with the help of AI from the public OBDex definition (CC0). Each make may use the code differently. Updated: 2026-11-10