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The evaporative (EVAP) emission control system is designed to trap and hold volatile organic compounds (VOCs), such as carbon monoxide (CO) and nitrogen oxide (NOx). The EVAP system prevents these gasoline vapors from being released into the atmosphere while reducing the vehicle’s exhaust emissions.

The EVAP system has a vent valve that allows fresh air to enter the system. If the powertrain control module (PCM) detects a fault with the circuit controlling the vent or the system is venting when it shouldn’t, a code P0499 will be logged by on-board diagnostics (OBD).

What Does the P0499 Code Mean?

mechanic inspecting the underside of a vehicle
The EVAP system prevents these gasoline vapors from being released into the atmosphere while reducing the vehicle’s exhaust emissions.

Diagnostic trouble code (DTC) P0499 stands for “Evaporative Emission System Vent Valve Control Circuit High.”

How an Evaporative Emission System Works

The EVAP system consists of hoses, valves, and a charcoal canister. To prevent vapors from being released into the atmosphere, they are routed into the charcoal canister, then into the intake airflow where they are burned in the engine.

As the canister purge solenoid valve opens, the manifold vacuum draws in the vapors from the canister. The same vacuum is applied to the pressure control valve. As the valve opens, fumes from the fuel tank are routed into the canister until it reaches the engine.

Once the valve closes, the pressure control valve is shut to keep vapors from escaping into the atmosphere.

engine control module activating P0499 code
Once the PCM detects any problem with the EVAP system’s vent valve, it will log a P0499 code.

The vent valve is normally open and is closed whenever the PCM tests the system. Because of the valve’s location, it is often subject to rust and corrosion. Once the PCM detects any problem with the EVAP system’s vent valve, it will log a P0499 code.

Note: The definition of code P0499 could be different depending on the vehicle manufacturer. Consult the appropriate repair manual or repair database for the exact code definition.

What are the Common Causes of the P0499 Code?

  • EVAP vent valve failure
  • Wiring issues
  • PCM failure

What are the Common Symptoms of the P0499 Code?

How to Diagnose the P0499 Code?

P0499 is commonly stored in makes like Dodge, Honda, and Chevrolet, among others. While this DTC is considered a generic powertrain code, keep in mind that the steps for diagnosis and repair can vary depending on your vehicle’s specifications. If you’re not familiar with the process of diagnosing an EVAP system, it’s a good idea to bring your vehicle to a certified mechanic instead.

Here are some videos that show what the process might involve:

How to Fix the P0499 Code

Fixing a P0499 code can be frustrating and confusing if you don’t have the right tools and know-how for the job. Under these circumstances, it’s best to bring your vehicle to the nearest repair shop to ensure that the steps for repair are being followed correctly.

If you’re a seasoned DIYer who knows your way around trouble codes, you’ll know the importance of reading the repair manual before proceeding. Guides from those like Chilton or an ALLDATA subscription contain vehicle-specific repair information, so you might want to check those out before you start.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic.

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