Thermostat Rationality

P0128 - THERMOSTAT RATIONALITY





When Monitored and Set Condition

- When Monitored: With the engine running.
- Set Condition: The PCM predicts a coolant temperature value that it will compare to the actual coolant temperature. If the two coolant temperature values are not within 10°C (18°F) of each other, this DTC is set.





For a complete Powertrain Control Module Circuit Diagram, Schematics and Diagrams.

Diagnostic Test

1. PRE-DIAGNOSTIC CHECK OUT

NOTE: Always perform diagnostics with a fully charged battery.

NOTE: Check connectors - Clean/repair as necessary. Poor pin to terminal connections can set DTCs.

NOTE: Check for applicable TSBs related to the problem.

Turn the ignition on.
With the scan tool, read PCM DTCs.
Using the wiring diagram/schematic as a guide, inspect the wiring and connectors. Repair as necessary.
Perform this procedure prior to symptom diagnosis.

Continue

- Go To 2

2. CHECK FOR CURRENT DTC
NOTE: If any ECT, CMP, IAT or CKP sensor DTCs have set along with P0128, diagnose them first before continuing.

NOTE: If the PCM detects and stores a DTC, the PCM also stores the engine/vehicle operating conditions under which the DTC was set. Some of these conditions are displayed on the scan tool at the same time the DTC is displayed.

NOTE: Before erasing stored DTCs, record these conditions. Attempting to duplicate these conditions may assist when checking for an active DTC.

Turn the ignition on.
With the scan tool, erase PCM DTCs.

NOTE: It maybe necessary to road test the vehicle for this DTC to set.

With the scan tool, read PCM DTCs.
Did this DTC set again?

Yes

- Go To 3

No

- Go To 15

3. CHECKING THE COOLANT LEVEL AND CONDITION
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 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. Ensure 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.

Check the coolant level and condition.
Is the coolant level and condition OK?

Yes

- Go To 4

No

- Locate and repair any coolant leaks and add the necessary amount of coolant. Service and Repair.
- Perform POWERTRAIN VERIFICATION TEST - VER 2. Powertrain Verification Test

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

With the ignition on.
With the scan tool, read the ENG COOLANT TEMP 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 ENG COOLANT TEMP value. The temperature 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 ENG COOLANT TEMP value in the scan tool 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 5

No

- Replace the Thermostat. Engine Coolant Thermostat - Removal.
- Perform POWERTRAIN VERIFICATION TEST - VER 2. Powertrain Verification Test

5. INTAKE AIR TEMP SENSOR OPERATION
With the ignition on.
With the scan tool, read and record the AIR INTAKE TEMP value
Using a temperature probe, measure the ambient air temperature near the IAT sensor.
Is the AIR INTAKE TEMP value within 3°C (5°F) of the temperature probe reading?

Yes

- Go To 6

No

- Go To 8

6. ECT SENSOR OPERATION
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.

With the ignition on.
With the scan tool, read and record the ENG COOLANT TEMP value
Using a temperature probe, measure the engine coolant temperature.
Is the ENG COOLANT TEMP value within 3°C (5°F) of the temperature probe reading?

Yes

- Go To 7

No

- Go To 8

7. OTHER POSSIBLE CAUSES
Inspect the Temperature sensors for any physical damage.
Inspect the engine coolant. Ensure the coolant is at the proper level. Service and Repair.
Ensure the Temperature sensors are properly mounted.
Ensure the CMP and CKP sensors are mounted properly. Check the connectors for any signs of damage.

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.

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.
Inspect and clean all PCM, engine, and chassis grounds.
Were any problems found during the above inspections?

Yes

- Repair as necessary.
- Perform POWERTRAIN VERIFICATION TEST - VER 2. Powertrain Verification Test

No

- Go To 15

8. SIGNAL CIRCUIT SHORT TO VOLTAGE




NOTE: 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 ECT Sensor harness connector.

NOTE: Check connectors - Clean/repair as necessary.

With the ignition on.
Measure the voltage of the ECT Sensor Signal circuit at the ECT Sensor harness connector.
Is the voltage below 5.2 volts?

Yes

- Go To 9

No

- Repair the ECT Signal circuit for a short to voltage.
- Perform POWERTRAIN VERIFICATION TEST - VER 2. Powertrain Verification Test

9. MEASURE THE ENGINE COOLANT TEMPERATURE SENSOR VOLTAGE WITH THE SCAN TOOL
With the ignition on.
With the scan tool, read the COOLANT TEMP ADC voltage.
Is the voltage above 4.6 volts?

Yes

- Go To 10

No

- Go To 13

10. ECT SENSOR VOLTAGE BELOW 1.0 VOLT
Turn the ignition off.
Using a jumper wire, jumper across the ECT Sensor harness connector.
Turn the ignition on.
With the scan tool, read the COOLANT TEMP ADC voltage.
Is the voltage below 1.0 volt?

Yes

- Replace the Engine Coolant Temperature Sensor. Engine Coolant Temperature Sensor - Removal.
- Perform POWERTRAIN VERIFICATION TEST - VER 2. Powertrain Verification Test

No

- Go To 11

11. ECT SIGNAL SIGNAL CIRCUIT OPEN




Turn the ignition off.
Disconnect the PCM C3 harness connector.

NOTE: Check connectors - Clean/repair as necessary.

Measure the resistance of the ECT Signal circuit from the ECT Sensor harness connector to the PCM C3 harness connector.
Is the resistance below 5.0 ohms?

Yes

- Go To 12

No

- Repair the ECT Signal circuit for an open.
- Perform POWERTRAIN VERIFICATION TEST - VER 2. Powertrain Verification Test

12. SENSOR GROUND CIRCUIT OPEN




With the ignition off.
Measure the resistance of the Sensor Ground circuit from the ECT Sensor harness connector to the PCM C3 harness connector.
Is the resistance below 5.0 ohms?

Yes

- Replace and program the Powertrain Control Module.
- Perform POWERTRAIN VERIFICATION TEST - VER 2. Powertrain Verification Test

No

- Repair the Sensor Ground circuit for an open.
- Perform POWERTRAIN VERIFICATION TEST - VER 2. Powertrain Verification Test

13. ECT SENSOR SIGNAL CIRCUIT SHORT TO GROUND




Turn the ignition off.
Measure the resistance between ground and the ECT Sensor Signal circuit.
Is the resistance above 100 kohms?

Yes

- Go To 14

No

- Repair the ECT Signal circuit for a short to ground.
- Perform POWERTRAIN VERIFICATION TEST - VER 2. Powertrain Verification Test

14. ECT SENSOR SIGNAL CIRCUIT SHORT TO SENSOR GROUND CIRCUIT




With the ignition off.
Measure the resistance between the Sensor Ground and the ECT Sensor Signal circuit at the ECT Sensor harness connector.
Is the resistance above 100 kohms?

Yes

- Replace and program the Powertrain Control Module.
- Perform POWERTRAIN VERIFICATION TEST - VER 2. Powertrain Verification Test

No

- Repair the Sensor Ground circuit for a short to the ECT Signal circuit.
- Perform POWERTRAIN VERIFICATION TEST - VER 2. Powertrain Verification Test

15. INTERMITTENT WIRING AND CONNECTORS
The condition that caused this DTC to set is currently not present. Inspect the related wiring harness for a possible intermittent condition.

NOTE: Check connectors - Clean/repair as necessary. Poor pin to terminal connections can set DTCs.

Using the wiring diagram/schematic as a guide, inspect the wiring and connectors specific to this DTC. Wiggle the wires while checking for shorts and open circuits.

NOTE: Check for any Technical Service Bulletins that may apply.

Were there any problems found?

Yes

- Repair as necessary.
- Perform POWERTRAIN VERIFICATION TEST - VER 2. Powertrain Verification Test

No

- The condition that caused this DTC to set is currently not present. Inspect the related wiring harness for a possible intermittent condition.