The P0499 code is a diagnostic trouble code (DTC) related to the Evaporative Emission Control System (EVAP) in your vehicle. It specifically points to an issue with the circuit controlling the vent valve or solenoid, indicating that the circuit has a higher than expected voltage. This might sound complicated, but essentially, it means there’s an electrical problem with the part of your car that deals with harmful vapors from the fuel system.
P0499 quick overview
| Meaning | P0499:”EVAP System Vent Valve/Solenoid Circuit High.” |
| Is it serious? | Yes, it can affect the vehicle’s emissions system and potentially its overall performance. |
| Possible causes | – Faulty EVAP canister vent solenoid – Open or short in the vent valve/solenoid circuit – Poor electrical connection in the vent valve/solenoid circuit – Faulty or damaged ECM (Engine Control Module) |
| How to diagnose? | Check grounds and electrical connections for integrity. Investigate electrical supply to the vent solenoid, including voltage and ground. Inspect wiring for damage, especially under the battery or in areas prone to corrosion. Perform a voltage drop test on the canister vent solenoid circuit. Test solenoid functionality using a handheld vacuum pump and monitoring power with a noid light. Verify ECM operation, considering replacement if necessary. |
P0499 Meaning
When your car’s computer throws a P0499 code, it’s telling you there’s an issue with the system that controls fuel vapors from the gas tank. Specifically, the problem lies in the part that vents these vapors into the engine to be burned off. If something’s wrong here, like a bad vent valve or a wiring issue, it could lead to your car emitting more pollution than it should. It can also cause other symptoms, like trouble starting your engine or a check engine light that won’t go off. Fixing it might involve checking the electrical parts for damage or replacing the vent valve itself.
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How to fix P0499 – learn from mechanics
I looked into P0499 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.
Case #1: Jeep
Vehicle Information 2005 Jeep Wrangler SE 2.4L
Issue + Repair History
- No start issue after installing a new battery.
- Initial diagnostic revealed codes: P0499 and P0688.
- ECM not energizing ASD relay or engaging starter relay in crank mode.
- Replaced ECM with factory unit.
Other OBD2 Codes + Test Results
- P0688: ASD sense circuit low
- P0499: NVLD canister vent circuit high
Mechanic Suggestions
- Check grounds thoroughly.
- Investigate if jump-starting occurred.
- Verify ASD sense line voltage to PCM.
Final Fix The NVLD shorted out, potentially causing a spike in the ECM. The ECM was replaced under warranty and properly programmed. Additionally, the NVLD was replaced, resolving the issue.
Case #2: Acura
Vehicle Information 2003 Acura MDX Touring 3.5L
Issue + Repair History
- MIL illuminated with DTC P0455 (Evap large leak).
- Identified fuel filler neck leaking, replaced it along with the cap.
- MIL illuminated again, this time with DTC P0499 (EVAP CVS Circuit Voltage High).
Other OBD2 Codes + Test Results
- P0499: EVAP CVS Circuit Voltage High
Mechanic Suggestions
- Perform voltage drop test on the canister vent solenoid circuit.
- Test for power and ground at the CVS valve; if not present, diagnose further.
- Verify the integrity of control wiring and PCM functionality.
- Use a handheld vacuum pump to test the solenoid’s functionality.
- In areas prone to rust, check for solenoid body corrosion affecting vacuum sealing.
- Test solenoid by verifying power, using a noid light, and monitoring ground command from the PCM.
Final Fix The faulty component was identified as the canister vent solenoid, despite being located on the firewall instead of the canister itself. Replacing the vent solenoid resolved the issue, with a salvaged part used for the replacement.
Case #3: Dodge
Vehicle Information 2004 Dodge Dakota 3.7L
Issue + Repair History
- MIL illuminated with DTC P0499 (NVLD Canister Vent Valve Solenoid Circuit High).
- Functional test for NVLD showed solenoid inoperative.
- NVLD receiving 9.3 volts consistently, even when the functional test is off.
- Previous repair attempted with NVLD replacement by another shop.
Other OBD2 Codes + Test Results
- P0499: NVLD Canister Vent Valve Solenoid Circuit High
Mechanic Suggestions
- Check for acid damage to wiring under the battery, possibly affecting circuitry.
- Disconnect headlight switch to check for wiring cross or short circuits.
- Try resetting PCM by disconnecting the battery for several minutes.
- Ensure proper voltage supply (B+) to the solenoid, check ignition switch and power circuit.
- Verify ground integrity and voltage levels at NVLD, noting expected readings for solenoid activation.
Final Fix Discovered aftermarket DRL system installed, affecting vehicle electronics. Upon removal, identified defective ECM. However, customer declined repairs.
How to diagnose P0499
I made you an overview of solving this DTC, based on all the advice that was suggested by certified mechanics to address this issue in previously mentioned service cases.
| Diagnostic Step | Explanation |
|---|---|
| Check grounds and electrical connections | Ensure all connections are secure and free of corrosion for proper function. |
| Investigate electrical supply to the vent solenoid | Verify the solenoid receives correct voltage and ground. |
| Inspect wiring for damage | Look for any signs of wear, acid damage, or corrosion, especially under the battery or in rust-prone areas. |
| Perform a voltage drop test on the canister vent solenoid circuit | Helps identify any resistance issues that could affect the solenoid’s operation. |
| Test solenoid functionality with a handheld vacuum pump | This confirms whether the solenoid can open and close properly. |
| Verify ECM operation | A faulty ECM can misinterpret signals or fail to control the vent solenoid correctly. |
Each of these steps offers a targeted approach to diagnosing and fixing the underlying issues related to the P0499 code, guiding both mechanics and vehicle owners through the troubleshooting process.
Car Diagnostics Training
- Car Electrical basics
- OBD-II scanner basics
- How to use an OBD reader
- Introduction to a Bi-directional Scan tool
- OBD scanner buyer’s guide + discounts
I test OBD-II scanners and make DIY Engine diagnostics guides to help you solve your car problems without having to depend on the mechanic. A lot of them will try to scam you or are just no help at all. About Juraj Lukacko