For a complete wiring diagram Refer to Section 8W.The PCM predicts what the engine coolant temperature should be, based on the engine coolant temperature at start-up, ambient temperature and how the vehicle is subsequently driven. The predicted engine coolant temperature is compared to the Engine Coolant Temperature Sensor reading. The error between the two is calculated and integrated with respect to time. When the Thermostat diagnostic runs, the integrated error is compared to a calibrated threshold and pass/fail is determined. Separate pass and fail thresholds are used in order to improve accuracy of the diagnostic.When Monitored:Engine running.Set Condition:This diagnostic will fail when the model temperature reaches 91° C (196° F) before the ECT. reaches 80° C (176° F. This monitor only runs after a cold start condition ECT, ACT, AMB are all within 10° C (50° F. of one another. Two Trip Fault. Three good trips to turn off the MIL.Possible CausesLOW COOLANT LEVELTHERMOSTAT OPERATIONSIGNAL CIRCUIT SHORTED TO BATTERY VOLTAGETEMPERATURE SENSORSIGNAL CIRCUIT OPENSENSOR GROUND CIRCUIT OPENSIGNAL CIRCUIT SHORTED TO GROUNDSIGNAL CIRCUIT SHORTED TO THE (K900), (G931) SENSOR GROUND CIRCUITOTHER POSSIBLE CAUSESPCMAlways perform the Pre-Diagnostic Troubleshooting procedure before proceeding. (Refer to 9 - ENGINE - DIAGNOSIS AND TESTING).1. ACTIVE DTCIf any ECT, AAT, CMP or CKP sensor DTCs have set along with P0128, diagnose them before continuing.Make sure that the Pinion Factor has been programmed correctly into the PCM.Diagnose any CAN - C Communication DTCs before continuing.Start the engine.Allow the engine to reach normal operating temperature.With a scan tool, select View DTCs.It may be necessary to drive the vehicle to meet the conditions to set this DTC, try to repeat the conditions in which the fault originally set by reviewing the Freeze Frame data.Is the DTC Active or Pending 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. Make sure 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.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 including extreme burns, scalding, or death. Allow the engine to cool before opening the cooling system.Check the coolant system to make sure that the coolant is in good condition and at the proper level.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. THERMOSTAT OPERATION This test works best if performed on a cold engine (cold soak).Ignition on, engine not running.With a scan tool, read the ECT 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 ECT 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 scan tool, ECT Temperature value 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. AMBIENT AIR TEMP SENSOR OPERATIONIgnition on, engine not running.With a scan tool, read and record the AAT Sensor Temperature value.Using the DRB Temperature Probe #CH7050, or an equivalent temperature measuring tool, measure the ambient air temperature near the AAT Sensor.Is the AAT Sensor value with -15°C (5°F) of the temperature probe reading?YesGo To 5NoGo To 75. ECT SENSOR OPERATIONMake sure the engine cooling system is cool before removing the pressure cap or any hose. The cooling system is pressurized when hot. Failure to follow these instructions can result in personal injury including extreme burns, scalding, or death.With a scan tool, read and record the ECT Sensor Temperature value.Use the DRB Temperature Probe #CH7050, or an equivalent temperature measuring tool, measure the engine coolant temperature.Is the ECT Sensor value with -15°C (5°F) of the temperature probe reading?YesGo To 6NoGo To 76. OTHER POSSIBLE CAUSESInspect the Temperature Sensors for any physical damage.Inspect the engine coolant. Make sure the coolant is at the proper level. Refer to the Service Information COOLING.Make sure the Temperature Sensors are properly installed.Make sure the CMP and CKP sensors are installed properly. Check the connectors for any signs of damage.When the engine is operating, do not stand in direct line with the fan. Do not put your hands near the pulleys, belts, or fan. Do not wear loose clothing. Failure to follow these instructions can result in personal injury or death. Refer to any Technical Service Bulletins (TSBs) that may apply.With the engine running at normal operating temperature, monitor the Temperature sensor parameters while wiggling the wire harness. Look for parameter values to change.Visually inspect the related wire harness. Look for any chafed, pierced, pinched, partially broken wires and broken, bent, pushed out, or corroded terminals.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.Inspect and clean all PCM, engine, and chassis grounds.Were any problems found during the above inspections?YesRepair as necessary.Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)NoTest Complete, the conditions that set this DTC are not present at this time. 7. SIGNAL CIRCUIT SHORTED TO BATTERY VOLTAGE

Visually inspect both the component and the PCM connectors. Look for damage, partially broken wires and backed out or corroded terminalsTurn the ignition off.Disconnect the applicable Temperature Sensor harness connector.Disconnect the C4 TIPM harness connector and the C2 PCM harness connector.Ignition on, engine not running.Measure the voltage on the (K2) ECT Signal circuit and the (G31) AAT Signal circuit at the appropriate Temperature Sensor harness connector.Is the voltage above 5.2 volts?YesRepair the short to battery voltage in the Signal circuit.Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)NoGo To 88. TEMPERATURE SENSOR

Turn the ignition off.Connect the C4 TIPM harness connector and the C2 PCM harness connector.Connect a jumper wire across the ECT and AAT Sensor harness connectors.Ignition on, engine not running.With a scan tool, read the Temperature Sensor voltage.Does the voltage start at 5.0 volts and drop below 1.0 volt when the jumper wire is installed?YesReplace the appropriate Temperature Sensor.Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)NoGo To 9Disconnect the jumper wire before continuing.9. 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 Sensor Signal circuit from the ECT Sensor harness connector to the appropriate terminal of special tool #8815.Measure the resistance of the (G31) AAT Sensor Signal circuit from the AAT Sensor harness connector to the C4 TIPM harness connector.Is the resistance below 5.0 ohms?YesGo To 10NoRepair the open in the Sensor Signal circuit.Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)10. SENSOR GROUND CIRCUIT OPEN

Measure the resistance of the (K900) Sensor ground circuit from the appropriate Temperature Sensor harness connector to the appropriate terminal of special tool #8815.Measure the resistance of the (G931) Sensor ground circuit from the AAT Sensor harness connector to the C4 TIPM harness connector.Is the resistance below 5.0 ohms?YesGo To 11NoRepair the open in the Sensor ground circuit.Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)11. SIGNAL CIRCUIT SHORTED TO GROUND

Measure the resistance between ground and the Sensor Signal circuit in the Temperature Sensor harness connector.Is the resistance below 100 ohms?YesRepair the short to ground in the Signal circuit.Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)NoGo To 1212. SIGNAL CIRCUIT SHORTED TO THE (K900) SENSOR GROUND CIRCUIT OR THE (G931) SENSOR GROUND CIRCUIT

Measure the resistance between the Signal circuit and the (K900) Sensor ground circuit in the (K2) ECT Sensor harness connector.Measure the resistance between the Signal circuit and the (G931) Sensor ground circuit in the (G31) AAT Sensor harness connector.Is the resistance below 100 ohms?YesRepair the short between the Sensor ground circuit and the Sensor Signal circuit.Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)NoGo To 13 13. 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)