Error Code P0499 is described as Evaporative Emission System Vent Valve Control Circuit High. This means that a part of the EVAP control system is not working properly, which can be caused by clogged, broken or faulty valves.

This code is a generic trouble code, meaning it applies to all vehicles equipped with OBD-II system, particularly vehicles made since 1996 up to present. Specifications on the definition, troubleshooting and repairs may vary from one make or model to another.


The engine’s EVAP system is composed on various components working together, including (but not limited to) fuel lines, gas cap, purge valve, charcoal canister, system pressure sensor and flow sensor, connectors and electrical wirings, vacuum lines, fuel vapor hoses and fuel tank.

The PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes) oversees the evaporative emission control system. The second it detects a variation in the voltage reading that does not coincide with the reference voltage indicated by the manufacturer, it will set the code indicating a fault. In the case of Error Code P0499, the fault refers to the circuit that controls the vent for EVAP emission system vapor.

This fault happens when the fuel sits in the tank and the fuel vapor escapes; while driving, the vent opens, allowing the vapors to be circulated into the engine intake manifold, charcoal canister or right into the atmosphere, depending on the vehicle.

Common Symptoms

Error Code P0499 doesn’t come with drivability issues. It’s most obvious symptom, and many times only symptom, is the illumination of the Check Engine light. There are however, other emission codes stored in the PCM.

Possible Causes

Error Code P0499 can be caused by multiple factors, but in many cases, the problem is caused by abnormal pressure in the evaporative emissions system vent valve control circuit due to improperly installed fuel cap. Other possible causes include:

  • Defective EVAP canister vent valve
  • Defective purge control solenoid or valve
  • Defective pressure sensor
  • Broken or clogged charcoal canister
  • Cracked, broken vacuum or fuel vapor hoses
  • Blown fuse, which can be sign of a more serious problem

How to Check

After the OBD-II scanner detects the fault code, technicians proceed on their diagnosis by conducting a visual inspection of the connectors, wirings and other electrical components of the emission system. Any damaged on any elements must be repaired, reconnected, if not replaced,

Mechanics should also conduct a rested of the system to clear the codes from the PCM and see if the code appears again. If the code reappears, then that means there’s really an issue in the EVAP system and problematic components must be repaired, if not replaced,

How to Fix

Based on the diagnosis, there are several ways to fix this code, including:

  • Proper installation of the fuel cap
  • Replacement of the defective EVAP canister vent valve
  • Replacement of faulty purge control solenoid or valve
  • Replacement of broken charcoal canister
  • Repair or replacement of broken or cracked fuel vapor hoses
  • Replacement of faulty pressure sensor
  • Replacement of blown fuses (note that this repair may indicate there’s a bigger problem on the vehicle that must be addressed)

After each step of repairs, it is important to clear the codes from the PCM and retest the system. If the code doesn’t reappear, then the last repair must have resolved the issue. This process narrows down the issues and makes it easier and faster to repair any future issues.

Most people commonly misdiagnose this error code and replace a completely fine EVAP emission control system, rather than checking the vacuum for leaks, or the fuel cap if it’s properly installed.

Also, this error code is a mild issue and will not affect the vehicle’s operation. However, as with many EVAP emission control system trouble codes, this code will cause the vehicle to fail emission test.