P0446: EVAP System Vent Control Circuit

What does P0446 mean?

The evaporative emission (EVAP) system stores fuel vapour from the tank in a charcoal canister; a purge valve lets the engine draw it off, and a vent valve opens the canister to fresh air. The vent valve is normally open and closes for the leak test. By its wording P0446 is its control circuit, and Ford uses it that way for open and shorted circuits. GM, however, uses P0446 for vent system performance – a blocked vent path: while purging with the vent valve open, the tank must not build up vacuum; more than 2989 Pa for 5 s before 4 litres have been purged sets the code, MIL after two trips. GM reports vent circuit faults as P0449, P0498 and P0499. A blocked vent makes refuelling difficult.

The code tells you which monitor failed, not which part is broken. Test before replacing parts.

Symptoms

• Check engine light on
• Refuelling problems, nozzle clicks off early (blocked vent)
• Hissing when opening the fuel cap (vacuum)
• EVAP leak test does not run

Common causes

• High Vent valve or filter blocked by dirt, dust or water (GM reading)
• Medium Vent valve electrically faulty, coil open or shorted (Ford reading)
• Medium Connector or wire to the vent valve corroded or open
• Medium Canister clogged with charcoal or liquid (overfilling)
• Low Vent hose kinked or blocked by an insect nest

Likelihood is a general estimate across vehicles; your make and model may differ.