A L L Diagnostic Trouble Codes ( DTC ) |Testing and...

P0128-THERMOSTAT RATIONALITY

For a complete wiring diagram refer to: Electrical Diagrams

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.

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Possible Causes

- LOW COOLANT LEVEL

- THERMOSTAT OPERATION

- SIGNAL CIRCUIT SHORTED TO BATTERY VOLTAGE

- TEMPERATURE SENSOR

- SIGNAL CIRCUIT OPEN

- (K900) SENSOR GROUND CIRCUIT OPEN

- SIGNAL CIRCUIT SHORTED TO GROUND

- SIGNAL CIRCUIT SHORTED TO (K900), (G931) SENSOR GROUND

- OTHER POSSIBLE CAUSES

- PCM

Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding.

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.

Perform POWERTRAIN VERIFICATION TEST. See: A L L Diagnostic Trouble Codes ( DTC ) > Verification Tests >

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.


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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.



3. THERMOSTAT OPERATION

NOTE: This test works best if performed on a cold engine (cold soak).


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 degrees 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.



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


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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. 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.

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.

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.

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.



No >> Test Complete.

7. SIGNAL CIRCUIT SHORTED TO BATTERY VOLTAGE


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


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and backed out or corroded terminals.

Turn the ignition off.

Disconnect the applicable Temperature Sensor harness connector.

Disconnect the applicable controller harness connectors.


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.



No >> Go To 8

8. TEMPERATURE SENSOR


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Connect the controller harness connectors.

Connect a jumper wire across Sensor harness connectors.


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.



No >> Go To 9

NOTE: Disconnect the jumper wire before continuing.

9. SIGNAL CIRCUIT OPEN


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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 of the Sensor Signal circuit Sensor harness connector to 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.



10. SENSOR GROUND CIRCUIT OPEN


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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.



11. SIGNAL CIRCUIT SHORTED TO GROUND


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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.



No >> Go To 12

12. SIGNAL CIRCUIT SHORTED TO SENSOR GROUND CIRCUIT


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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.



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



No >> Replace applicable Control Module per Service Information.




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