P0128

P0128-THERMOSTAT RATIONALITY





For a complete wiring diagram Consult Diagrams/Electrical.

- When Monitored:
Engine running.

- Set Condition:
The PCM predicts a coolant temperature value that it will compare to the actual coolant temperature. A significant difference results in an error. Two Trip Fault. Three good trips to turn off the MIL.





Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. Initial Inspection and Diagnostic Overview.

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.

Ignition on, engine not running.
With a scan tool, read DTCs.
Is the DTC active at this time?

Yes

- Go To 2

No

- Refer to the INTERMITTENT CONDITION Diagnostic Procedure.Intermittent Condition
- Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification 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 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?

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).

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.

NOTE: 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.

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.

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
Ignition 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?

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 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?

Yes

- Go To 6

No

- Go To 7

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

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 personal injury or death.

Search for 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.

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.

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.

7. SIGNAL CIRCUIT SHORTED TO BATTERY VOLTAGE

NOTE: Visually inspect both the component and the PCM/FCM connectors. Look for damage, partially broken wires and backed out or corroded terminals

Turn the ignition off.
Disconnect the applicable Temperature Sensor harness connector.
Disconnect the applicable controller harness connectors.
Ignition on, engine not running.
Measure the voltage on the Signal circuit at the Temperature Sensor harness connector.
Is voltage present?

Yes

- Repair the short to battery voltage in the Signal circuit.
- Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test

No

- Go To 8

8. TEMPERATURE SENSOR
Turn the ignition off.
Connect the controller harness connectors.
Connect a jumper wire across 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?

Yes

- 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
Turn the ignition off.

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.

Measure the resistance between the Sensor Signal circuit at the Sensor harness connector and the appropriate terminal of special tool #8815 or the FCM.
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
Measure the resistance of the Sensor ground circuit from the appropriate Temperature Sensor harness connector to the appropriate terminal of special tool #8815 or the FCM.
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
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 SENSOR GROUND CIRCUIT
Measure the resistance between the Signal circuit and the Sensor ground circuit in the 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. FCM/PCM

NOTE: Before continuing, check the FCM/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?

Yes

- Repair as necessary.
- Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test

No

- Replace applicable Control Module.
- Perform the POWERTRAIN VERIFICATION TEST. Powertrain Verification Test