For a complete wiring diagram Refer to Section 8W.When Monitored:Ignition on, Engine running, and no ECT DTCs present.Set Condition:PCM recognizes that the ECT has failed its self coherence test. The coolant temp should only change at a certain rate, if this rate is too slow or too fast this fault will set. Two trip fault. Three good trips to clear MIL. The MIL and ETC light will illuminate.Possible CausesLOW COOLANT LEVEL(K2) ECT SIGNAL CIRCUIT SHORTED TO BATTERY VOLTAGE(K2) ECT SIGNAL CIRCUIT OPEN(K900) SENSOR GROUND CIRCUIT OPEN(K2) ECT SIGNAL CIRCUIT SHORTED TO GROUND(K2) ECT SIGNAL CIRCUIT SHORTED TO THE (K900) SENSOR GROUND CIRCUITTHERMOSTATECT SENSORPCMAlways perform the Pre-Diagnostic Troubleshooting procedure before proceeding. (Refer to 9 - ENGINE - DIAGNOSIS AND TESTING).1. ACTIVE DTCIgnition on, engine not running.If this code sets during extreme ambient temperatures, improper installation of a block heater could be the cause of this DTC.With a scan tool, select View DTCs.Diagnose all other ECT and Cooling System codes before continuing.Is the DTC active at this time?YesGo To 2NoRefer to the INTERMITTENT CONDITION Diagnostic Procedure.Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)2. LOW COOLANT LEVELIf an Engine Coolant Temperature (ECT) DTC is set along with this code, diagnose the ECT DTC first.Inspect the ECT terminals and related PCM terminals. Ensure the terminals are free from corrosion and damage.The best way to diagnose this DTC is to allow the vehicle to sit overnight outside in order to have a totally cold soaked engine.Extremely cold outside ambient temperatures may have caused this DTC to set.Need to make sure that no Cooling System DTCs are set or changes that would make the warm up much slower or much faster: broken water pump can set this, addition of aftermarket auxiliary cooler can set this DTC.Never open the cooling system when the engine is hot. The system is under pressure. Failure to follow these instructions can result in personal injury or death. Allow the engine to cool before opening the cooling system.Inspect the coolant system for proper level and condition.Is the coolant level and condition OK?YesGo To 3NoInspect the vehicle for a coolant leak and add the necessary amount of coolant.Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)3. THERMOSTATThis test works best if performed on a cold engine (cold soak).Ignition on, engine not running.With a scan tool, read the Eng Coolant Tmp Deg value. If the engine was allowed to sit overnight (cold soak), the temperature value should be a sensible value that is somewhere close to the ambient temperature.If engine coolant temperature is above 82° C (180° F), allow the engine to cool until 65°C (150°F) is reached.Start the Engine.During engine warm-up monitor the Eng Coolant Tmp Deg value. The temp deg value change should be a smooth transition from start up to normal operating temp 82° C (180° F) . Also monitor the actual coolant temperature with a thermometer.As the engine warms up to operating temperature, the actual coolant temperature (thermometer reading) and the Eng Coolant Tmp Deg on the scan tool should stay relatively close to each other.Using the appropriate service information, determine the proper opening temperature of the thermostat.Did the thermostat open at the proper temperature?YesGo To 4NoReplace the thermostat.Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)4. ECT SENSOR

Connect a jumper between the (K2) ECT Signal circuit and the (K900) Sensor ground circuit in the ECT Sensor harness connector.Turn the ignition off.Disconnect the ECT Sensor harness connector.With a scan tool, read the ECT voltage.Is the voltage below 1.0 volt?YesReplace the ECT Sensor.Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)NoGo To 55. (K2) ECT SIGNAL CIRCUIT SHORTED TO BATTERY VOLTAGE

Turn the ignition off.Disconnect the C2 PCM harness connector.Ignition on, engine not running.Measure the voltage on the (K2) ECT Signal circuit in the ECT Sensor harness connector.Is the voltage above 0 volts?YesRepair the short to battery voltage in the (K2) ECT Signal circuit.Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)NoGo To 66. (K2) ECT SIGNAL CIRCUIT OPEN

Turn the ignition off.Do not probe the PCM harness connectors. Probing the PCM harness connectors will damage the PCM terminals resulting in poor terminal to pin connection. Install Miller Special Tool #8815 to perform diagnosis.Measure the resistance of the (K2) ECT Signal circuit from the ECT Sensor harness connector to the appropriate terminal of special tool #8815.Is the resistance below 5.0 ohms?YesGo To 7NoRepair the open in the (K2) ECT Signal circuit.Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)7. (K900) SENSOR GROUND CIRCUIT OPEN

Measure the resistance of the (K900) Sensor ground circuit from the ECT Sensor harness connector to the appropriate terminal of special tool #8815.Is the resistance below 5.0 ohms?YesGo To 8NoRepair the open in the (K900) Sensor ground circuit.Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)8. (K2) ECT SIGNAL CIRCUIT SHORTED TO GROUND

Measure the resistance between ground and the (K2) ECT Signal circuit in the ECT Sensor harness connector.Is the resistance below 100 ohms?YesRepair the short to ground in the (K2) ECT Signal circuit.Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)NoGo To 99. (K2) ECT SIGNAL CIRCUIT SHORTED TO THE (K900) SENSOR GROUND CIRCUIT

Measure the resistance between the (K2) ECT Signal circuit and the (K900) Sensor ground circuit in the ECT Sensor harness connector.Is the resistance below 5.0 ohms?YesRepair the short between the (K900) Sensor ground and the (K2) ECT Signal circuit.Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)NoGo To 1010. PCMBefore continuing, check the PCM harness connector terminals for corrosion, damage, or terminal push out. Repair as necessary.Using the schematics as a guide, inspect the wire harness and connectors. Pay particular attention to all Power and Ground circuits.Were there any problems found?YesRepair as necessary.Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)NoReplace and program the Powertrain Control Module per Service Information.Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)