P0499 is set when the Powertrain Control Module (PCM) detects a high voltage condition in the evaporative emission (EVAP) system vent valve control circuit. This indicates a short-to-power fault, a failed vent valve, or wiring issues preventing proper circuit control.
The vent valve internal circuit has failed, causing a short-to-power condition that pulls the control circuit high.
Damaged wiring or corroded connector in the vent valve circuit has created a short to battery voltage, forcing the circuit high.
Water intrusion or corrosion in the vent valve connector can bridge circuits and cause a high voltage reading at the PCM.
Some vehicles use a relay to control the vent valve; a stuck relay can supply constant power to the circuit.
Rare internal PCM issue causing incorrect voltage reference to the vent valve circuit; this is the least likely cause and should be diagnosed only after all other possibilities are ruled out.
No. The PCM is almost never the cause of this code. In the vast majority of cases, the vent valve itself or its wiring has failed. Technicians sometimes recommend PCM replacement because the code points to a 'circuit high' condition, but this is almost always incorrect and unnecessary.
Yes, the vehicle will typically drive normally with no drivability issues. However, the EVAP system will not function properly, which may cause you to fail an emissions test.
The Check Engine Light may turn off temporarily if the fault is intermittent, but the code will typically return. The underlying electrical fault will not resolve on its own and should be diagnosed to prevent potential fuel tank venting issues.