Diagnostic Aids

1999 GMC Forward Control 4.3 W, StandardSECTION Diagnostic Aids

Using FREEZE FRAME and/or FAILURE RECORDS data may aid in locating an intermittent condition. If DTC cannot be duplicated, this information can be useful in determining how many times since the DTC set. The Fail Counter and Pass Counter can also be used to determine how many ignition cycles the diagnostic reported a pass and/or a fail. Operate vehicle with the same FREEZE FRAME conditions (RPM, load, vehicle speed, temperature, etc.) that were noted.

If other DTC(s) are set that share the same ground and/or 5-volt reference circuit, check for faulty connections or wiring. If engine has been allowed to sit overnight, ECT sensor and IAT sensor values should display within a few degrees of each other.

If engine coolant temperature exceeds 140°F (60°C), this indicates that the engine is capable of reaching proper temperature, but not necessarily in the correct amount of time. This test must be repeated on a cold engine, engine coolant and intake air temperature at less than 122°F (50°C) and within 5°F (3°C) of each other, and the time required to reach the temperature threshold must be measured.

The engine should reach the specified temperature within 8 minutes. If specified temperature is not reached within the specified time, check the cooling 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.