P0128
P0128-THERMOSTAT RATIONALITY
For a complete wiring diagram, refer to the Wiring Information.
Theory of Operation
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:
With the engine running, ambient temperature between -8° C (17.6° F) and 50° C (122° F), start up coolant temperature less than 50° C (122° F), and average vehicle speed greater than 16 kph (10 mph) until coolant temperature reaches 85° C (185° F).
- Set Condition:
The PCM detects that the actual engine coolant temperature falls too far below the predicted engine coolant temperature and the predicted coolant temperature reaches the predicted target value before the actual coolant temperature reaches the actual coolant temperature target value. Two trip fault. Three good trips to turn off the MIL.
Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. Pre-Diagnostic Troubleshooting Procedure.
1. ACTIVE DIAGNOSTIC TROUBLE CODE (DTC)
NOTE: If any ECT, AAT, CMP or CKP sensor DTCs have set along with P0128, diagnose them before continuing.
NOTE: Make sure that the Pinion Factor has been programmed correctly into the PCM.
1. Start the engine.
2. Allow the engine to reach normal operating temperature.
3. With a scan tool, select View DTCs.
NOTE: 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?
Yes
- Go To 2
No
- Perform the INTERMITTENT CONDITION diagnostic procedure. Intermittent Condition Test.
2. LOW COOLANT LEVEL
NOTE: If an Engine Coolant Temperature (ECT) DTC is set along with this code, diagnose the ECT DTC first.
NOTE: Inspect the ECT terminals and related PCM terminals. Make sure the terminals are free from corrosion and damage.
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.
NOTE: Extremely cold outside ambient temperatures may have caused this DTC to set.
WARNING: 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 fatal injury. Allow the engine to cool before opening the cooling system.
1. 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?
Yes
- Go To 3
No
- Inspect the vehicle for a coolant leak and add the necessary amount of coolant.
- Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test.
3. THERMOSTAT OPERATION
NOTE: This test works best if performed on a cold engine (cold soak).
1. Ignition on, engine not running.
2. 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.
NOTE: If engine coolant temperature is above 82° C (180° F), allow the engine to cool until 65° C (150° F) is reached.
3. Start the Engine.
4. 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.
NOTE: 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.
5. Using the appropriate service information, determine the proper opening temperature of the thermostat.
Did the thermostat open at the proper temperature?
Yes
- Go To 4
No
- Replace the thermostat.
- Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test.
4. AMBIENT AIR TEMP SENSOR OPERATION
1. Ignition on, engine not running.
2. With a scan tool, read and record the AAT Sensor Temperature value.
3. Using a suitable temperature measuring tool, measure the ambient air temperature near the AAT Sensor.
Is the AAT Sensor value within 3° C (5° F) of the reading?
Yes
- Go To 5
No
- Go To 7
5. ECT SENSOR OPERATION
WARNING: Make 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 possible serious or fatal injury.
1. With a scan tool, read and record the ECT Sensor Temperature value.
2. Using a suitable temperature measuring tool, measure the engine coolant temperature.
Is the ECT Sensor value within 3° C (5° F) of the reading?
Yes
- Go To 6
No
- Go To 7
6. OTHER POSSIBLE CAUSES
WARNING: 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 possible serious or fatal injury.
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 PCM Pinout Box 8815 to perform diagnosis.
1. Inspect the Temperature Sensors for any physical damage.
2. Inspect the engine coolant. Make sure the coolant is at the proper level. Refer to the Service Information COOLING.
3. Make sure the Temperature Sensors are properly installed.
4. Make sure the CMP and CKP sensors are installed properly. Check the connectors for any signs of damage.
5. Perform any Technical Service Bulletins (TSBs) that may apply.
6. With the engine running at normal operating temperature, monitor the Temperature sensor parameters while wiggling the wire harness. Look for parameter values to change.
7. Visually inspect the related wire harness. Look for any chafed, pierced, pinched, partially broken wires and broken, bent, pushed out, or corroded terminals.
8. Inspect and clean all PCM, engine, and chassis grounds.
Were any problems found during the above inspections?
Yes
- Repair as necessary.
- Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test.
No
- Test Complete, the conditions that set this DTC are not present at this time.
7. SIGNAL CIRCUIT SHORTED TO VOLTAGE
NOTE: Visually inspect both the component and the PCM connectors. Look for damage, partially broken wires and backed out or corroded terminals
1. Turn the ignition off.
2. Disconnect the applicable Temperature Sensor harness connector.
3. Disconnect the C2 FCM harness connector and the C2 PCM harness connector.
4. Ignition on, engine not running.
5. Measure the voltage on the (K2) ECT Signal circuit and the (G31) AAT Signal circuit at the appropriate Temperature Sensor harness connector.
Is there any voltage present?
Yes
- Repair the short to voltage in the Signal circuit.
- Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test.
No
- Go To 8
8. TEMPERATURE SENSOR
1. Turn the ignition off.
2. Connect the C2 FCM harness connector and the C2 PCM harness connector.
3. Ignition on, engine not running.
4. With a scan tool, read the Temperature Sensor voltage.
5. Connect a jumper wire across the ECT and AAT Sensor harness connectors.
Does the voltage start at 5.0 Volts and drop below 1.0 Volt when the jumper wire is installed?
Yes
- Verify that there is good pin to terminal contact in the Sensor and Control Module connectors. If OK, replace the appropriate Temperature Sensor.
- Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test.
No
- Go To 9
NOTE: Disconnect the jumper wire before continuing.
9. SIGNAL CIRCUIT OPEN
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 PCM Pinout Box 8815 to perform diagnosis.
1. Turn the ignition off.
2. Measure the resistance of the (K2) ECT Sensor Signal circuit from the ECT Sensor harness connector to the appropriate terminal of PCM Pinout Box 8815.
3. Measure the resistance of the (G31) AAT Sensor Signal circuit from the AAT Sensor harness connector to the C2 FCM harness connector.
Is the resistance below 5.0 Ohms?
Yes
- Go To 10
No
- Repair the open in the Sensor Signal circuit.
- Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test.
10. SENSOR GROUND CIRCUIT OPEN
1. Measure the resistance of the (K900) Sensor ground circuit from the appropriate Temperature Sensor harness connector to the appropriate terminal of PCM Pinout Box 8815.
2. Measure the resistance of the (G931) Sensor ground circuit from the AAT Sensor harness connector to the C2 FCM harness connector.
Is the resistance below 5.0 Ohms?
Yes
- Go To 11
No
- Repair the open in the Sensor ground circuit.
- Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test.
11. SIGNAL CIRCUIT SHORTED TO GROUND
1. Measure the resistance between ground and the Sensor Signal circuit in the Temperature Sensor harness connector.
Is the resistance below 100 Ohms?
Yes
- Repair the short to ground in the Signal circuit.
- Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test.
No
- Go To 12
12. SIGNAL CIRCUIT SHORTED TO THE (K900) SENSOR GROUND CIRCUIT OR THE (G931) SENSOR GROUND CIRCUIT
1. Measure the resistance between the Signal circuit and the (K900) Sensor ground circuit in the (K2) ECT Sensor harness connector.
2. 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?
Yes
- Repair the short between the Sensor ground circuit and the Sensor Signal circuit.
- Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test.
No
- Go To 13
13. POWERTRAIN CONTROL MODULE (PCM)
1. Using the wiring diagram/schematic as a guide, inspect the wiring and connectors between the Temperature Sensors and the Powertrain Control Module (PCM).
2. Look for any chafed, pierced, pinched, or partially broken wires.
3. Look for broken, bent, pushed out or corroded terminals. Verify that there is good pin to terminal contact in the Sensor and Powertrain Control Module connectors.
4. Perform any Technical Service Bulletins that may apply.
Were there any problems found?
Yes
- Repair as necessary.
- Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test.
No
- Replace and program the Powertrain Control Module.
- Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test.