Diagnostic Aids

1999 GMC Forward Control 4.3 W, StandardSECTION Diagnostic Aids

An intermittent problem can be caused by misrouted harness, rubbed through wire insulation or broken wire inside insulation. Monitor scan tool while moving related connectors and wiring harness. Whenever a fault is detected, scan data will change from ON to OFF or from OFF to ON. Moving gear selector slowly through each gear while monitoring scan tool may also help isolate problem.

Using FREEZE FRAME and/or FAILURE RECORDS data mode may aid in locating an intermittent condition. If DTC cannot be duplicated, information included in FREEZE FRAME and/or FAILURE RECORDS data can be useful in determining how many miles since DTC set. FAIL COUNTER and PASS COUNTER can also be used to determine how many ignition cycles diagnostic reported a pass or a fail condition. To isolate when DTC failed, operate vehicle within same freeze frame conditions (RPM, load, vehicle speed, temperature, etc.).

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When to See a Mechanic

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

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