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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 139 (ENGINE CONTROL SYSTEM - SELF-DIAGNOSTICS)DIAGNOSTIC TESTSDTC P0128: ECT BELOW THERMOSTAT REGULATING TEMPERATUREDIAGNOSTIC PROCEDURES
1999 GMC Forward Control 5.7 R, Automatic

Diagnostic Procedures

1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Procedures
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.
  1. Did you perform the Diagnostic System Check - Engine Controls? If yes, go to next step. If no, perform diagnostic system check - engine controls. See DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS  under SELF-DIAGNOSTIC SYSTEM.
  2. Allow engine to cool to ambient temperature. Turn ignition switch to RUN position, engine OFF. Compare ECT sensor temperature reading to IAT sensor temperature reading on a scan tool. Are ECT and IAT readings within 2 or 3 degrees of each other? If yes, go to next step. If no, go to step  6.
  3. Start the engine and immediately observe ECT parameter on the scan tool. Continue to observe ECT while idling the engine for 10 minutes. Does ECT indicated on scan tool increase by more than 36°F (20°C) within 10 minutes? If yes, go to step  5. If no, go to next step.
  4. Are any ECT sensor DTCs set? If yes, go to DTC P0115: ECT SENSOR CIRCUIT  or DTC P0116: ECT SENSOR CIRCUIT PERFORMANCE  . If no, go to step  8.
  5. Fault is not present. Are there any DTCs stored that have not been diagnosed? If yes, go to DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, go to DIAGNOSTIC AIDS .
  6. Measure resistance of ECT sensor with a DMM. Compare measured resistance of ECT sensor with resistance values in Temperature vs. Resistance table. See TEMPERATURE VS. RESISTANCE  table. Is resistance of ECT sensor within specifications? If yes, go to next step. If no, go to step  9.
  7. Check for any of the following conditions:
    • A high resistance in ECT sensor wiring.
    • A poor connection at ECT sensor.
    • A poor connection at PCM.
    Repair as necessary. Did you find and correct the condition? If yes, go to step  11. If no, go to step  10.
  8. Check engine cooling system for all of the following:
    • Correct coolant level.
    • Correct operation of cooling system.
    • Correct operation of thermostat.
    Repair as necessary. After repair, go to step  11.
  9. Replace ECT sensor. After replacing sensor, go to step  11.
  10. Replace PCM. After replacing PCM, go to step  11.
  11. Use scan tool to clear DTCs. Turn ignition switch to OFF position for 30 seconds. Start engine. Operate vehicle within CONDITIONS FOR RUNNING DTC . Does DTC run and pass? If yes, go to next step. If no, go to step  2.
  12. With a scan tool, observe stored information, CAPTURE INFO. Does scan tool display any DTCs that have not diagnosed? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, system is okay at this time.
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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.