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Home >> GMC >> 2003 >> Safari Van Cargo, AWD >> Repair and Diagnosis >> External Pages >> Different car >> Section 89 (Engine Control System Self-Diagnostics - 3.1L Engine) >> Diagnostic Tests >> DTC P0128: Engine Coolant Thermostat >> Diagnostic Procedures

Diagnostic Procedures

WARNING: This page is about a different car, the 2002 Pontiac Grand Prix, 2002 Chevrolet Malibu, and 2002 Buick Century. However, it is still accessible from the selected car via links, so may be relevant.
  1. Perform diagnostic system check - engine controls. See DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS  under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
    NOTE: The PCM will enable the engine cooling fans when certain Engine Coolant Temperature (ECT) diagnostic trouble codes are set.
  2. Is the cooling system low on coolant? If yes, check and repair cause of coolant loss. If no, go to next step.
  3. Turn ignition off. Disconnect ECT sensor. Connect a 3-amp fused jumper wire between the signal circuit and the low reference circuit of the ECT sensor harness connector. Turn ignition on, with engine off. Observe the ECT sensor temperature with a scan tool. Does scan tool indicate that ECT sensor temperature is more than 280°F (138°C)? If yes, go to step  7. If no, go to next step.
  4. Turn ignition off. Disconnect ECT sensor. Connect a 3-amp fused jumper wire between the signal circuit of the ECT sensor harness connector and a known-good ground. Turn ignition on, with engine off. Observe the ECT sensor temperature with a scan tool. Does scan tool indicate that ECT sensor temperature is more than 280°F (138°C)? If yes, go to next step. If no, go to step  6.
  5. Test ECT sensor reference low circuit for a high resistance or an open. Repair the circuit as necessary. After repairs, go to step  10.
  6. Test ECT sensor signal circuit for a high resistance or an open. Repair the circuit as necessary. After repairs, go to step  10.
  7. Remove ECT sensor. Suspend the ECT sensor and a thermometer in a pan of water. Place pan on a burner or hot plate. Test the resistance of the ECT sensor through the specified range of 50-194°F (10-90°C). Does the resistance and temperature closely match? See appropriate SENSOR RESISTANCE table in appropriate SENSOR OPERATING RANGE CHARTS article. If yes, go to next step. If no, go to step  9.
  8. Reinstall ECT sensor. After replacing sensor, diagnose thermostat operation.
  9. Replace ECT sensor. After replacing sensor, go to next step.
  10. Clear DTCs with scan tool. Turn ignition off for 30 seconds. Start engine. Operate the vehicle within the Conditions for Running the DTC. See CONDITIONS FOR RUNNING DTC . Does the DTC run and pass? If yes, go to next step. If no, see DIAGNOSTIC AIDS .
  11. Using scan tool, observe stored information and Capture Info. Does the scan tool display any DTCs that you have not diagnosed? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, system is okay.
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