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DTC P0128: Thermostat Rationality: Testing

WARNING: Never open the cooling system when the engine is hot. The system is under pressure. Extreme burns or scalding may result. Allow the engine to cool before opening the cooling system.
NOTE: If any Engine Coolant Temperature (ECT) sensor, Ambient Air Temperature (AAT) sensor, Camshaft Position (CMP) sensor or Crankshaft Position (CKP) sensor DTCs have set along with P0125, diagnose them first before continuing. Ensure that pinion factor has been programmed correctly into the PCM.
  1. Ignition on, engine not running. With the DRBIII® scan tool, read DTCs and record the related freeze frame data. Is the Good Trip displayed and equal to zero? If yes, go to next step. If no, see INTERMITTENT CONDITION  under SELF-DIAGNOSTIC SYSTEM.
    2. WARNING: Never open the cooling system when the engine is hot. The system is under pressure. Extreme burns or scalding may result. Allow the engine to cool before opening the cooling system.
    NOTE: If a Engine Coolant Temperature (ECT) DTC is set along with this code, diagnose the ECT DTC first.
    NOTE: 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. Inspect the ECT terminals and related PCM terminals. Ensure the terminals are free from corrosion and damage. Extremely cold outside ambient temperatures may have caused this DTC to set.
  2. 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? If yes, go to next step. If no, inspect the vehicle for a coolant leak and add the necessary amount of coolant.
  3. This test works best if performed on a cold engine (cold soak). Ignition on, engine not running. With the DRBIII® 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 180°F (82°C), allow the engine to cool until 150°F (65°C) 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 180°F (82°C). 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 ECT Deg in the DRBIII® scan tool sensor should stay relatively close to each other. The proper opening temperature of the thermostat is 192-199°F (88-93°C). Did the thermostat open at the proper temperature? If yes, go to next step. If no, replace the thermostat.
  4. Turn the ignition on. With the DRBIII® scan tool, read and record the AAT sensor temperature value Using the DRB temperature probe (CH7050), measure the AAT near the AAT sensor. Is the AAT sensor value with 5°F (-15°C) of the temperature probe reading? If yes, go to next step. If no, go to step  7.
    5. WARNING: Make sure the engine cooling system is cool before removing the pressure cap or any hose. Severe personal injury may result from escaping hot coolant. The cooling system is pressurized when hot
  5. Turn the ignition on. With the DRBIII® scan tool, read and record the ECT sensor temperature value. Using the DRB temperature probe (CH7050), measure the engine coolant temperature. Is the ECT sensor value with 5°F (-15°C) of the temperature probe reading? If yes, go to next step. If no, go to step  7.
    6. WARNING: When the engine is operating, do not stand in a direct line with the fan. Do not put your hands near the pulleys, belts or fan. Do not wear loose clothing.
    CAUTION: 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.
  6. Inspect the temperature sensors for any physical damage. Inspect the engine coolant. Ensure the coolant is at the proper level. Ensure the temperature sensors are properly mounted. Ensure the CMP and CKP sensors are mounted properly. Check the connectors for any signs of damage. 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. Inspect and clean all PCM, engine, and chassis grounds. Were any problems found during the above inspections? If yes, repair as necessary. If no, test complete.
  7. Visually inspect both the component and the PCM connectors. Look for damage, partially broken wires and backed out or corroded terminals Turn the ignition off. Disconnect the applicable temperature sensor harness connector. Ignition on, engine not running. Measure the voltage of the signal circuit in the applicable temperature sensor harness connector. Is the voltage above 5.2 volts? If yes, repair the short to battery voltage in the signal circuit. If no, go to next step.
  8. Turn the ignition off. Disconnect the applicable temperature sensor harness connector. Ignition on, engine not running. With the DRBIII® scan tool, read the temperature sensor voltage. Is the voltage above 4.6 volts? If yes, go to next step. If no, go to step  12.
  9. Turn the ignition off. Disconnect the applicable temperature sensor harness connector. Using a jumper wire, jumper across the temperature sensor harness connector. Ignition on, engine not running. With the DRBIII® scan tool, read the temperature voltage. Is the voltage below 1.0 volt? If yes, replace the applicable temperature sensor. If no, go to next step.
    10. CAUTION: 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.
  10. Turn the ignition off. Disconnect the applicable temperature sensor harness connector. Disconnect the PCM harness connectors. Measure the resistance of the signal circuit from the temperature sensor harness connector to the appropriate terminal of Miller Special Tool (8815). Is the resistance below 5.0 ohms? If yes, go to next step. If no, repair the open in the signal circuit.
    11. CAUTION: 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.
    NOTE: Before continuing, check the PCM harness connector terminals for corrosion, damage, or terminal push out. Repair as necessary.
  11. Turn the ignition off. Disconnect the applicable temperature sensor harness connector. Disconnect the PCM harness connectors. Measure the resistance of the sensor ground circuit from the AAT sensor harness connector to the appropriate terminal of Miller Special Tool (8815). Is the resistance below 5.0 ohms? If yes, replace and program the PCM. See PROGRAMMING . If no, repair the open in the sensor ground circuit.
  12. Turn the ignition off. Disconnect the applicable temperature sensor harness connector. Disconnect the PCM harness connectors. Measure the resistance between ground and the signal circuit in the temperature harness connector. Is the resistance below 100 ohms? If yes, repair the short to ground in the signal circuit. If no, go to next step.
    13. NOTE: Before continuing, check the PCM harness connector terminals for corrosion, damage or terminal push out. Repair as necessary.
  13. Turn the ignition off. Disconnect the applicable temperature sensor harness connector. Disconnect the PCM harness connectors. Measure the resistance between the signal circuit and the sensor ground circuit in the temperature sensor harness connector. Is the resistance below 100 ohms? If yes, repair the sensor ground shorted to the signal circuit. If no, if there are no possible causes remaining, replace and program the PCM. See PROGRAMMING .
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