Powertrain, generic · Auxiliary emission controls

P0446 EVAP System Vent Control Circuit

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

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.

Likely causes

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

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

Repair estimate

DifficultyMediumDIY possible
Parts and labour€40–400
Time0.5–2 h

Typical range across vehicles; the real cost depends on what the testing finds.

What to check first

  1. Check vent valve
  2. Inspect vent filter
  3. Check wiring

Parts involved

evap vent valvevent filtercharcoal canisterengine control module

Related codes

P0447P0448P0449P0498P0499P0455

References: fordservicecontent.com, newtonnet.co.uk, gsi.ext.gm.com, gsi.ext.gm.com.