P1646
EVAP Vent Solenoid Output Circuit
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The P1646 code is a GM-specific code indicating a problem with the electrical circuit that controls the EVAP vent solenoid. The PCM uses an internal Output Driver Module (ODM) to control this solenoid — when the output should be OFF, it expects to see 12 volts, and when ON, it expects near 0 volts. If the voltage doesn't match what's expected, the fault line is tripped.
The EVAP vent solenoid is located near the charcoal canister, typically at the rear of the vehicle near the fuel tank. It controls the flow of fresh air into the EVAP system. Because of its exposed location, wiring corrosion and connector damage are common failure points. The solenoid itself can also fail electrically.
This code typically causes no driveability symptoms — you'll only notice the check engine light. However, it will cause the vehicle to fail an emissions test. The vent solenoid is an inexpensive part ($20–$60), and replacement is usually straightforward. The main challenge for DIYers is accessing the part, as it requires getting under the rear of the vehicle.
Severity
Symptoms
- •Check engine light is on
- •Faint gasoline smell near the vehicle
- •No noticeable change in how the vehicle drives
- •May fail emissions testing
- •Slight rough idle in rare cases
Likely Causes
The vent solenoid valve has failed electrically, causing the PCM's output driver module to detect incorrect voltage on the control circuit — either staying high when commanded low, or vice versa.
A short to ground, short to power, or open circuit in the wiring between the PCM and the vent solenoid causes the output driver module to flag a fault.
The vent solenoid is typically located near the rear of the vehicle by the fuel tank, where it is exposed to road spray, salt, and debris. Connector corrosion is common.
The PCM's internal output driver chip that controls the vent solenoid has failed. This is less common but can occur, especially on higher-mileage vehicles.
Estimated Cost
Professional Repair
Includes parts + labor
Common Fixes
- Replace the EVAP vent solenoid valve
- Inspect and repair wiring to the vent solenoid
- Clean corroded connectors at the vent solenoid
- Check the vent solenoid fuse and relay