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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 139 (ENGINE CONTROL SYSTEM - SELF-DIAGNOSTICS)DIAGNOSTIC TESTSDTC P0446: EVAP VENT SYSTEM PERFORMANCEDIAGNOSTIC AIDS
1999 GMC Forward Control 5.7 R, Automatic

Diagnostic Aids

1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Aids
WARNING: This page is about a different car, the 2003 Pontiac Vibe. However, it is still accessible from the selected car via links, so may be relevant.
  • If DTC P0446 is the only EVAP DTC set, the most likely cause is an inoperable EVAP canister vent solenoid, or pressure switching solenoid. Check EVAP solenoids carefully for faulty connections and erratic operation.
  • An intermittent malfunction may be caused by a fault in any of EVAP solenoid electrical circuits. Check wiring harness and components for an intermittent condition.
  • If DTC cannot be duplicated, information included in freeze frame data can be useful in determining vehicle operating conditions when DTC was first set.
  • Operating vehicle in CHECK MODE will increase ability of PCM module to detect engine control system malfunctions. See SERVICE BAY TESTS  under SELF-DIAGNOSTIC SYSTEM.
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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.