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P0128






P0128-THERMOSTAT RATIONALITY

For a complete wiring diagram, refer to the Wiring Information.

Theory of Operation

The Powertrain Control Module (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 monitor 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 is not at a calibrated value when the pass/fail evaluation point is reached. Two trip fault. Three good trips to turn off the MIL.





Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. Pre-Diagnostic Troubleshooting Procedure.

Diagnostic Test

1. ACTIVE 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 the 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: 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. Component Tests and General Diagnostics.

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
1. Ignition on, engine not running.
2. With the scan tool, read the ECT Sensor Degree value. If the engine was allowed to sit overnight (cold soak), the temperature value should be a 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 Sensor Degree value. The temp degree 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. Engine Coolant Thermostat - Removal.
- Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test.

4. AMBIENT AIR TEMP SENSOR OPERATION
1. Ignition on, engine not running.
2. With the scan tool, read and record the AAT Sensor Temperature value.
3. Monitor the actual ambient air temperature near the AAT Sensor with a thermometer.

Is the AAT Sensor value within -15°C (5°F) of the thermometer reading?

Yes

- Go To 5

No

- Replace the Ambient Air Temperature Sensor.
- Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test.

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 the scan tool, read and record the ECT Sensor Temperature value.
2. Monitor the actual coolant temperature with a thermometer.

Is the ECT Sensor value with -15°C (5°F) of the thermometer reading?

Yes

- Go To 6

No

- Replace the Engine Coolant Temperature Sensor.
- Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test.

6. OTHER POSSIBLE CAUSES
1. Inspect the Temperature Sensors for any physical damage.
2. Inspect the engine coolant. Make sure the coolant is at the proper level.
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.

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 the PCM Pinout Box to perform diagnosis.

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

- Go To 7

7. 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.

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