P0499

Code

Generic P — Powertrain

EVAP System Vent Valve Control Circuit High

Brand: Generic

Type: P — Powertrain

Views: UK: 21 EN: 39 RU: 33

AI status

Completed

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Page language: EN

Causes

Short to battery voltage (B+) on vent valve control circuit
Faulty EVAP vent/vent control valve (stuck or internal short)
Damaged, corroded or loose connector or pin at valve or PCM
Open or damaged wiring with intermittent contact or incorrect routing
Faulty PCM/ECM driver or internal module fault
Aftermarket equipment or previous repairs altering wiring

Symptoms

Malfunction Indicator Lamp (MIL) / Check Engine Light illuminated
Failed EVAP system readiness or failed emissions test
Unable to complete EVAP diagnostic (EVAP monitor may not run)
Occasional fuel vapour smell near fuel tank or canister (less common)
Possible drivability generally not affected

What to check

Read freeze-frame and live data with a scan tool; confirm P0499 is current and note freeze-frame conditions
Visually inspect vent valve, harness, connector for corrosion, damage, or water intrusion
Check related fuses and any inline relays for the EVAP circuit
Backprobing: measure voltage at vent valve connector with ignition ON and while commanding valve ON/OFF from scanner
Disconnect the valve and check harness side for short to B+ or abnormal voltage
Measure coil resistance of vent valve and compare to spec

Signal parameters

Typical wiring: one side usually has constant B+; PCM commonly switches ground (low-side). Behavior can vary by vehicle.
With ignition ON (valve off) expect battery voltage (~11–14 V) at the valve supply terminal; PCM control pin may read near battery or near supply depending on design.
When PCM commands valve ON (grounds the circuit), control terminal at harness should drop near 0–1 V relative to ground if low-side switching.
Coil current draw typical range: ~0.1–2.0 A (depends on valve); coil resistance typically tens to hundreds of ohms—compare to service spec.
P0499 triggers when the PCM sees higher-than-expected voltage on the control/feedback circuit compared to expected thresholds (vehicle specific).

Diagnostic algorithm

Verify the code with an OBD-II scanner, record freeze-frame, clear the code and attempt to reproduce.
Perform a visual inspection of the EVAP vent valve, canister area, wiring harness and connectors for damage, corrosion or water ingress.
Backprobe the vent valve connector: with ignition ON, record voltage at both terminals. Command the valve ON/OFF from the scanner and observe voltage changes.
Disconnect the vent valve. Measure voltage at the harness side. If voltage remains high (near B+) when PCM should be pulling low, suspect short to B+ or PCM driver fault.
Measure coil resistance of the vent valve. If open or outside spec, replace valve.
Check for short to B+ by isolating sections of wiring (disconnect harnesses, remove splices) and measure continuity to battery positive.
Inspect and test grounds and related power feeds. Repair damaged wires, corroded connectors or poor grounds.
If wiring and valve check good, test/replace PCM only after confirming wiring integrity (bench test or swap with known-good module only where appropriate and using manufacturer procedures).
After repairs, clear codes and perform drive cycle/EVAP monitor to verify the fault does not return.

Likely causes

Corroded or pushed-out pin at the vent valve connector causing backfeed to circuit
Wire chafe contacting a fused 12V source (short to B+)
Failed vent valve coil with abnormal internal resistance causing unusual voltage reading
PCM output transistor or monitoring circuit failure
Poor ground or harness damage near canister/valve

Fault status

⚠️ Status

PCM detected higher-than-expected voltage on the EVAP canister vent/vent valve control circuit. This indicates a potential short to battery voltage, failed vent valve, wiring/connector failure, or PCM driver fault that can prevent proper EVAP system operation and emissions testing.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1-3 hours