What causes fault code P0499?

• Most likely: Faulty EVAP vent valve (internal short or open circuit) • Likely: Corrosion or oxidation in the connector at the charcoal canister • Possible: Wiring harness break or short to voltage in the signal line • Rare: Faulty Engine Control Unit (ECU) or software issue

Powertrain (Engine & Gearbox)Standard EOBD

P0499 - Evaporative Emission System Vent Control Circuit High

Q: What are the symptoms of fault code P0499?

• Check Engine Light (MIL) illuminated • EVAP system monitors fail to complete • Potential difficulty refueling (gas pump constantly clicks off if valve is stuck closed) • Usually no noticeable driveability issues

Q: How do you fix fault code P0499?

STEP 1: Visually inspect the wiring and connector at the charcoal canister for corrosion or damage. Clean with contact cleaner if necessary. STEP 2: Check fuses related to the EVAP/Emission system. Measure for battery voltage (approx. 12V) at the connector with ignition ON. STEP 3: Measure the internal resistance of the vent valve using a multimeter. Normal resistance is typically between 15-30 Ohms. If OL (open) or 0 Ohms, replace the valve. STEP 4: Use a bidirectional scan tool to command the vent valve ON/OFF. If the voltage stays at 12V when commanded ON, check for an open ground circuit to the ECM. STEP 5: Replace the EVAP vent valve if the component fails electrical or functional tests.

Last updated:1 February 2026

Related:EVAP Vent Valve, Charcoal Canister, PCM/ECU, Wiring Harness

System

Powertrain (Engine & Gearbox)

Related Fault Codes

P0400

Exhaust Gas Recirculation Flow Malfunction

P0401

Exhaust Gas Recirculation (EGR) Flow Insufficient Detected

P0402

Exhaust Gas Recirculation Flow Excessive Detected

P0403

Exhaust Gas Recirculation (EGR) Control Circuit

Technical Description

P0499 is a diagnostic trouble code (DTC) that indicates the Engine Control Module (ECM) has detected a 'high' voltage condition in the Evaporative Emission (EVAP) vent valve control circuit. The vent valve is used to seal the EVAP system so the ECM can perform leak tests. When the ECM commands the vent valve ON (typically by providing a ground path), it expects to see the voltage drop. If the voltage stays high (near battery voltage), it indicates an open circuit, high resistance, or a short to power. This affects the vehicle's ability to run self-diagnostic leak tests on the fuel system.

Possible Causes

Faulty EVAP vent valve (internal short or open circuit)

Corrosion or oxidation in the connector at the charcoal canister

Wiring harness break or short to voltage in the signal line

Faulty Engine Control Unit (ECU) or software issue

Symptoms

Check Engine Light (MIL) illuminated

EVAP system monitors fail to complete

Potential difficulty refueling (gas pump constantly clicks off if valve is stuck closed)

Usually no noticeable driveability issues

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P0499 - Evaporative Emission System Vent Control Circuit High

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P0499 - Evaporative Emission System Vent Control Circuit High

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