What's the difference between P0499 and P0498?

Same circuit, opposite fault directions. P0499 is circuit high — the PCM sees voltage where it expected the control wire to be pulled down, which almost always means an open circuit, an unplugged connector, an open solenoid coil, or a short to power. P0498 is circuit low — the wire is pulled toward ground when it shouldn't be, pointing at a short to ground or a shorted winding.

Why do I have P0499 and a large EVAP leak code at the same time?

That combination is expected, not a coincidence. If the PCM can't drive the vent valve closed, the system stays open to atmosphere and can never hold vacuum, so any leak test that runs reports a large leak. Fix the P0499 circuit fault first, clear both codes, and then re-run the drive cycle. In many cases the leak code doesn't come back.

Can I keep driving with P0499?

Yes. The EVAP system doesn't affect combustion, so the engine runs normally with no safety concern. The consequences are the check engine light and a failed emissions readiness check, since the EVAP monitor can't complete while the vent circuit is faulted. Get it repaired before your next inspection, but there's no urgency beyond that.

Where is the EVAP vent valve located?

On most vehicles it's at the rear, at or very near the charcoal canister, mounted underneath the floor or on the frame rail close to the fuel tank. Some designs integrate it directly into the canister assembly. That under-vehicle location is exactly why corrosion, water intrusion, and rodent damage account for so many P0499 cases — check the connector and the surrounding harness carefully before ordering parts.

OBD-II trouble code

P0499: Evaporative Emission System Vent Valve Control Circuit High

The voltage on the EVAP vent valve control circuit is higher than the PCM expects — typically an open circuit, an unplugged connector, or a short to power. The engine drives normally, but the EVAP leak test can't run.

Low severityPowertrainEVAP SystemDrivable short-term

Quick facts

System: Powertrain
Category: EVAP System
Severity: Low severity
Drivable: Usually safe to drive short-term
Repair cost range: $0 – $450
DIY difficulty: Intermediate DIY

What does P0499 mean?

The EVAP vent valve controls whether the evaporative emissions system is open to the atmosphere or sealed. In normal operation it stays open, so the fuel tank can breathe as fuel is consumed and so the charcoal canister has a source of fresh air when the purge valve pulls vapor into the engine. The PCM closes it only to run a leak test, sealing the system so it can apply vacuum or pressure and watch whether the reading holds.

P0499 is the circuit-high electrical fault for that valve. The PCM drives the vent solenoid and monitors the control circuit voltage to verify the circuit responds. When it commands the valve and instead sees voltage sitting HIGH — up near battery voltage when the circuit should have been pulled toward ground — it sets P0499. The circuit isn't completing.

The overwhelmingly common causes are an open circuit or an unplugged connector. A vent valve connector that was disturbed during exhaust, fuel tank, or suspension work and never fully reseated is the classic one — the PCM sees an open circuit and immediately flags it. After that come broken control wires, an internally open solenoid coil (burned-out winding), a short to power that holds the circuit up, and corroded terminals that create enough resistance to look like an open. Because the vent valve is mounted under the vehicle near the charcoal canister, corrosion and rodent damage are far more likely than they would be on an underhood part.

What P0499 is not: a mechanical complaint. It says nothing about whether the valve physically opens or closes. That is what P0446 and P0448 describe. P0499 is only about what the PCM sees electrically on the control wire.

For the driver, nothing changes. The engine starts, idles, and drives identically, with no measurable effect on power or fuel economy. The cost is the check engine light plus a blocked EVAP readiness monitor — with the vent circuit faulted, the PCM will not attempt a leak test, so the monitor never sets to ready and the vehicle cannot pass an OBD-II readiness inspection. A vent valve that ends up stuck open also means the system can never seal, which on some vehicles produces a follow-on large-leak code.

Common causes

Unplugged, loose, or partially seated vent valve connector — very often disturbed during recent under-vehicle work
Open or broken control wire between the PCM and the vent valve
EVAP vent solenoid with an internally open (burned-out) coil winding
Corroded terminals in the vent valve connector creating high resistance that reads as an open
Control wire shorted to battery voltage
Rodent-chewed harness near the charcoal canister
Blown fuse on the EVAP supply circuit, leaving the circuit unable to pull down
Failed PCM driver on the vent control circuit (rare — verify everything else first)

Symptoms

Check engine light is on
EVAP readiness monitor won't complete, so the vehicle fails an emissions readiness check
No change in starting, idle, power, or fuel economy
Possible companion large-leak code (P0455 or P0442) if the valve is left stuck open and the system can never seal
Occasional faint fuel odor near the rear of the vehicle
No unusual noises or warning behavior beyond the light

Diagnostic steps

1.Scan and record all codes plus freeze-frame data. A large-leak code alongside P0499 is expected when the vent can't be commanded closed and does not necessarily mean a second, separate fault.
2.Go straight to the vent valve connector — usually at the rear of the vehicle near the charcoal canister — and confirm it is fully seated and latched. A disturbed connector is the single most common cause of this code.
3.Inspect the connector body and terminals for corrosion, green film, water intrusion, and backed-out pins.
4.Disconnect the valve and measure coil resistance. Most vent solenoids specify roughly 15 to 40 ohms; an open reading confirms a burned-out coil.
5.With the key on and the valve unplugged, verify battery voltage on the supply pin. No voltage means checking the EVAP fuse and the supply side of the harness.
6.Check the control wire for continuity from the valve connector back to the PCM connector. An open confirms a broken wire; a wire that shows battery voltage with the PCM disconnected indicates a short to power.
7.Inspect the harness run for chafe points and rodent damage, especially where it passes near the frame rail, exhaust, or fuel tank straps.
8.Command the vent closed with a bidirectional scan tool and listen for the click. Silence with an open coil reading confirms the solenoid; a click with the code still setting sends you back to the wiring.

Repair cost

$0 – $450

If the fix is reseating a connector that was left unplugged, the cost is nothing beyond the time to find it — always check that first. A replacement vent valve is typically $30 to $110 in parts with modest under-vehicle labor, putting a full replacement around $120 to $280. Wiring repairs run $80 to $300 depending on how much harness has to be opened up, and rodent damage on a long section near the canister can reach $450. PCM driver failure is uncommon and pushes the job past $600 with programming, so rule out everything else first.

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DIY vs shop

This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.