EVAP Vent Valve Control Circuit High
The P0499 code indicates that the Powertrain Control Module (PCM) has detected a high voltage condition in the control circuit of the evaporative emission (EVAP) system vent valve. This valve is responsible for sealing the EVAP system during leak testing. A high voltage reading typically points to a short to power, a faulty vent valve, or an issue with the wiring/connector. This can prevent the EVAP system from properly sealing, leading to failed emissions tests and potential fuel vapor leaks.
1. Most Common Causes
- Short to battery voltage (B+) in the EVAP vent valve control/signal wire
- Open circuit in the vent valve ground wire or the PCM's internal low-side driver
- EVAP vent valve solenoid with an open or internally shorted coil
- Damaged, corroded, or loose terminals in the vent valve connector
- Internal failure of the vent valve driver within the PCM
2. Symptoms You Will Notice
- Check Engine Light illuminated
- Failed emissions test
- Possible fuel odor
- No noticeable drivability issues in most cases
3. Step-by-Step Diagnosis
Step 1: Scan for codes and record freeze frame data.
Step 2: Visually inspect the EVAP vent valve and its wiring for damage, corrosion, or loose connections.
Step 3: Using a multimeter, check the vent valve control circuit for voltage with the ignition on. A high reading (near battery voltage) when the valve should be off indicates a short to power.
Step 4: Test the vent valve resistance (typically 20-40 ohms). Replace if out of spec.
Step 5: If wiring and valve are okay, check for a short to power in the PCM harness or a faulty PCM.
4. Frequently Asked Questions
Is it safe to drive with this code?▼
Yes, it is generally safe to drive, but the vehicle may fail an emissions test and there is a small risk of fuel vapor leaks. It should be repaired soon.
How much does it cost to fix?▼
The cost typically ranges from $50 to $150 for parts and labor, depending on whether the vent valve or wiring needs replacement.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.