Evaporative Emission System Vent Control Circuit High
Your car's evaporative emission system vent valve is stuck open or the circuit has excessive electrical resistance, like a clogged drain preventing proper airflow. The engine computer detected the vent control signal is too high, preventing the system from properly sealing the fuel tank.
The ECM monitors voltage on the evaporative vent control solenoid circuit, expecting specific on/off signal patterns and resistance levels. When the solenoid driver output remains high or circuit resistance exceeds thresholds, the fault is triggered.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Vent Control Solenoid Voltage | 0-5V switching pattern | Voltage remains above 4.5V continuously |
| Circuit Resistance | 6-14 ohms coil resistance | Resistance exceeds 20 ohms or open circuit |
Code P0499 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P0499 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.