EVAP canister vent valve codes

How its codes differ

• P0449 is the vent valve's electrical circuit: wiring, connector or solenoid coil.
• P0446 is the vent's performance: the circuit seems fine but the valve does not seal or the vent path is blocked.
• Because the leak test needs the vent closed, a vent that does not seal can set leak codes (P0455, P0442, P0456); GM has bulletins linking several of these codes to dust in the canister and vent, and to a vent solenoid that leaks only under vacuum, often when cold.

Codes

• P0446

  EVAP vent control performance problem

  Severity: Monitor

• P0449

  EVAP vent valve circuit fault

  Severity: Monitor

• P0455

  EVAP system large leak detected

  Severity: Monitor

• P0442

  EVAP system small leak detected

  Severity: Monitor

• P0456

  EVAP system very small leak detected

  Severity: Monitor

• P0440

  EVAP system fault (general)

  Severity: Monitor

Signs it's failing

• Check engine light with P0446, P0449 or a leak code
• Fuel pump nozzle clicking off early or a hard-to-fill tank when the vent is blocked
• Usually no change in how the car drives

How to test it

1. Inspect the vent valve's connector and hoses, and look for dirt or dust packed around the vent inlet, especially on vehicles driven on dirt roads. (Use caution)

   Warning: Fuel and fuel vapour are flammable. Work with the engine off and cool, in a ventilated area, with no smoking, sparks or flames nearby.

2. Commanding the vent closed with a scan tool and checking that it holds vacuum and pressure needs a capable scan tool or smoke machine; a shop can do this. (Best left to a shop)

   Warning: Fuel and fuel vapour are flammable. Work with the engine off and cool, in a ventilated area, with no smoking, sparks or flames nearby.

After the repair

• Allow several days of normal driving for the EVAP monitor to run again; it needs particular fuel level and temperature conditions, so it is often the last monitor to complete.
• Clear codes only after the repair, then drive normally over several days with both city and highway driving so the readiness monitors can complete; clearing codes or disconnecting the battery resets them to not ready, which fails an emissions test.