OBD-II Code P0499 — What It Means & How to Fix It

What Does This Error Mean?

P0499 means the PCM detected a high voltage condition in the EVAP canister vent control circuit. This is the opposite of P0498 — instead of too little voltage, there is too much or the circuit is floating open. A high-voltage condition in this circuit usually means the control wire is broken (open circuit) or shorted to battery power. The result is that the PCM cannot properly command the vent valve, disrupting EVAP system operation.

Affected Models

Most 1996 and newer vehicles with PCM-controlled EVAP vent valves
Common on GM, Ford, Chrysler, Toyota, Honda, Nissan, and Hyundai
Any vehicle with an electronically controlled canister vent solenoid
Vehicles where the vent solenoid is mounted in exposed underbody locations
Higher mileage vehicles with aging wiring harnesses prone to open circuits

Common Causes

Open circuit in the control wire to the vent solenoid — a broken wire reads as high voltage
Vent solenoid wiring connector has come unplugged or a pin has backed out of the connector
Control wire is shorted to battery voltage from contact with a power wire in the harness
Vent solenoid coil is open (burned out winding), causing the circuit to float high
PCM output driver for the vent valve circuit has failed open

How to Fix It

Check the EVAP vent solenoid connector first. Trace the wiring from the solenoid and verify the connector is fully seated. Unplug it, inspect for backed-out or spread pins, then reconnect firmly. Clear the code and retest.

A partially disconnected connector is one of the most common causes of high-voltage circuit codes because an open circuit floats to reference voltage.
Measure resistance across the vent solenoid terminals. A reading of infinite (open) indicates the solenoid coil is burned out. Most solenoids should read between 20 and 80 ohms. Replace the solenoid if it reads open.

Compare your reading to the service manual spec for your specific vehicle — solenoid resistance varies.
With the solenoid connector unplugged, probe the control wire and check for 12V battery voltage. There should be no battery voltage present on a PCM output wire with the ignition on. If you measure battery voltage, the wire has shorted to a nearby power wire.

A short to battery voltage on a PCM output is potentially damaging to the PCM. Repair the wiring short before connecting any new solenoid.
Flex and manipulate the wiring harness along its path from the solenoid to the PCM. Watch a multimeter connected to the circuit for any changes while moving the harness. An intermittent open will show as fluctuating readings when the broken section is moved.

Break points in wiring often occur at sharp bends, near grommets where the harness passes through the firewall, or at harness routing clips that have pulled the wire tight.
If a broken wire is found, repair it with a proper soldered and heat-shrink-sealed splice. Avoid using push-on connectors or bare wire twists in areas exposed to engine heat or moisture — they fail quickly and cause repeat problems.

Use adhesive-lined heat shrink tubing on any splice in the engine bay. It seals against moisture and holds the repair securely.

When to Call a Professional

Open-circuit faults in wiring are diagnosable with a multimeter and a wiring diagram. If the solenoid itself is open, replacement parts are inexpensive. Shops can typically locate and repair open circuit faults in 1 to 2 hours.

Frequently Asked Questions

What is the difference between P0498 and P0499?

They both refer to the EVAP vent control circuit but in opposite directions. P0498 is a low-voltage condition — typically a short to ground. P0499 is a high-voltage condition — typically an open circuit or short to power. Both codes prevent the PCM from properly controlling the vent valve.

Can a bad gas cap cause P0499?

No. A bad gas cap causes EVAP leak codes like P0440, P0455, or P0456 — not circuit codes like P0499. P0499 is an electrical circuit fault in the vent solenoid wiring, which is unrelated to the gas cap.

How long does it take for the EVAP monitor to reset after a repair?

The EVAP monitor typically requires several complete drive cycles to run. A drive cycle includes a cold start, highway driving, and idle time. In practice, most people need 3 to 7 days of normal driving before the EVAP monitor shows as complete (ready).