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DTC codes / P04xx · Auxiliary emission controls / P0499

P0499Evaporative Emission System Vent Valve Control Circuit High

P0499 means the powertrain control module (PCM) commanded the evaporative emission (EVAP) system vent valve to open or close, but the voltage on the valve's control circuit stayed higher than expected. The PCM monitors the vent valve circuit for a short to voltage or an open ground path, and it sets this code when the feedback voltage does not drop when the valve is energized.

SystemPowertrain
Code typeGeneric (all makes)
SeverityLow
Fix difficultyIntermediate
AreaAuxiliary emission controls

Symptoms you may notice

  • Illuminated check engine light, often with no other noticeable symptoms
  • A faint fuel or gasoline odor around the rear of the vehicle after parking
  • Difficulty or failure when the EVAP system runs its self-test, which may prevent other EVAP monitors from completing
  • In some vehicles, a slight hesitation or rough idle if the vent valve is stuck closed and tank pressure builds
  • No drivability changes in most cases because the vent valve only affects fuel vapor routing

Common causes, most likely first

  1. Damaged or chafed wiring between the vent valve and the PCM, especially where the harness passes near heat or moving suspension parts
  2. Corroded or loose electrical connector at the vent valve or at the PCM
  3. Failed vent valve solenoid with an internally shorted coil that holds the control circuit high
  4. Shorted control wire to a power source, such as a rubbed-through section touching a 12-volt feed
  5. Poor ground connection for the vent valve or for the PCM, causing the circuit to read high
  6. Faulty PCM driver for the vent valve circuit, though this is the least common cause

How to diagnose and fix it

  1. Inspect the vent valve connector and the PCM connector for corrosion, pushed pins, or loose terminals, and clean or repair as needed
  2. Disconnect the vent valve and measure resistance across its two terminals; compare to the manufacturer's specification, typically 20 to 30 ohms, and replace the valve if it is open or shorted
  3. With the valve disconnected and the key on, measure voltage on the control wire from the PCM; it should be near zero volts when the valve is commanded off, and a constant high reading points to a short to voltage
  4. Check the ground circuit for the vent valve with a voltage drop test while the circuit is loaded, repairing any high-resistance ground
  5. Wiggle-test the harness while watching live data for the vent valve command and voltage to catch intermittent wiring faults
  6. If the wiring and valve test good but the code returns, substitute a known-good vent valve and clear the code to see if it resets
  7. As a last step, verify the PCM driver with a bidirectional scan tool and, if the driver is confirmed faulty, replace or reprogram the PCM

Why the vent valve circuit reads high

  • A vent valve that is mechanically stuck closed can still pass a resistance test, so always command the valve open and closed with a scan tool and listen for the click before condemning the wiring.
  • The PCM often sets P0499 only after a failed EVAP monitor, so a vehicle with a weak battery or a recently cleared code may not show the fault until several drive cycles have completed.
  • Corrosion inside the vent valve connector is a frequent cause on vehicles that see road salt, and the corrosion can bridge the control circuit to the power feed inside the connector body.
  • A shorted vent valve coil can pull the entire EVAP circuit high and also set companion codes for the purge valve or tank pressure sensor, so diagnose the whole EVAP system rather than just the vent valve.
  • Some vehicles use a vent valve integrated into the charcoal canister, and replacing just the solenoid is not possible; the entire canister assembly must be replaced when the valve fails.

Can I keep driving with P0499?

The vehicle is safe to drive with P0499 because the vent valve only affects fuel vapor containment and does not change engine operation. Avoid topping off the fuel tank past the automatic shutoff and park away from open flames until the fault is repaired, since a stuck vent valve can allow fuel vapor to escape.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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