Buick Regal: DTC P0128

Diagnostic Instructions

• Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
• Review Strategy Based Diagnosis for an overview of the diagnostic approach.
• Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptor

DTC P0128

Engine Coolant Temperature (ECT) Below Thermostat Regulating Temperature

Diagnostic Fault Information

Typical Scan Tool Data

ECT Sensor

Circuit/System Description

The purpose of this diagnostic is to analyze the performance of the thermostat by using the engine coolant temperature (ECT) sensor to determine if the engine coolant will increase at the correct rate, and also to meet the calibrated target temperatures under various operating conditions.

The ECM uses the start-up ECT and the start-up intake air temperature (IAT) to begin the diagnostic calculation. The air flow into the engine is accumulated, and vehicle speed, distance, and engine run time are also factored in to determine if the ECT does increase normally and reach the calibrated target temperatures.

Conditions for Running the DTC

• DTCs P00B3, P00B6, P0101, P0102, P0103, P0111, P0112, P0113, P0116, P0117 or P0118 are not set.
• The engine OFF time is greater than 30 m.
• The start-up ECT sensor is between -40º to 66ºC (- 40º to 151ºF).
• Engine run time between 30 s and 30 m.
• The Ethanol in the fuel is less than 87 %.
• The vehicle has traveled greater than 0.8 kilometers (0.5 miles).
• The DTC runs once per ignition cycle when the above conditions are met.

Conditions for Setting the DTC

• The ECM determines that the ECT has not met its calibrated target of 71º C (160 ºF) when the start-up ambient temperature was between 10 and 52º C (50 and 126º F), for greater than 1 s.

  OR

• The ECM determines that the ECT has not met its calibrated target of 55º C (131 ºF) when the start-up ambient temperature was between - 7 and 10º C (19 and 50º F), for greater than 1 s.

Action Taken When the DTC Sets

• DTC P0128 is a Type B DTC.
• The cooling fans will be commanded ON.
• The AC compressor will be commanded OFF.

Conditions for Clearing the MIL/DTC

DTC P0128 is a Type B DTC.

Diagnostic Aids

• DTC P0128 occurring with insufficient vehicle interior heating is an indication of improper thermostat operation.
• A slight to moderate resistance in the ECT sensor signal circuit or low reference circuit will affect this diagnostic. This condition results in a greater voltage on the ECT sensor signal circuit, which is interpreted by the ECM as a colder ECT.

Reference Information

Schematic Reference

Engine Controls Wiring Schematics (LTG)

Connector End View Reference

Component Connector End View Index

Description and Operation

Cooling System Description and Operation

Electrical Information Reference

• Circuit Testing
• Connector Repairs
• Testing for Intermittent Conditions and Poor Connections
• Wiring Repairs

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

Circuit/System Verification

1. Ignition ON, verify that DTC P00B3, P00B6, P0101, P0102, P0103, P0111, P0112, P0113, P0116, P0117 or P0118 is not set.

• If DTC P00B3, P00B6, P0101, P0102, P0103, P0111, P0112, P0113, P0116, P0117 or P0118 is set

Refer to Diagnostic Trouble Code (DTC) List - Vehicle

• Go to next step: If DTC P00B3, P00B6, P0101, P0102, P0103, P0111, P0112, P0113, P0116, P0117 or P0118 is not set

2. Verify the cooling system is filled to the correct coolant level. Refer to Cooling System Draining and Filling (Static) or Cooling System Draining and Filling (GE 47716). If necessary, refer to Loss of Coolant.

• If the coolant level is not correct

Repair as necessary

• Go to next step: If the coolant level is correct

3. Engine running for 15 min with the A/C OFF, increase the engine speed to 3, 000 RPM.

4. Verify the scan tool ECT Sensor parameter is greater than 80ºC (176ºF).

• If less than 80ºC (176ºF)

Replace the Engine Coolant Thermostat.

• Go to next step: If greater than 80ºC (176ºF)

5. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.

6. Verify the DTC does not set.

• If the DTC sets

Refer to Circuit/System Testing.

• Go to next step: If the DTC does not set

7. All OK.

Circuit/System Testing

1. Ignition OFF and all vehicle systems OFF, disconnect the B34 Engine Coolant Temperature sensor harness connector at the sensor. It may take up to 2 minutes for all vehicle systems to power down.

2. Test for less than 5 Ω between the low reference circuit terminal A or 2 and ground.

• If 5 Ω or greater

1. Ignition OFF, disconnect the X2 harness connector at the K20 Engine Control Module.
2. Test for less than 2 Ω in the low reference circuit end to end.
   • If 2 Ω or greater, repair the open/high resistance in the circuit.
   • If less than 2 Ω, replace the K20 Engine Control Module.

• Go to next step: If less than 5 Ω

3. Ignition ON.

4. Verify the scan tool ECT Sensor parameter is colder than -39º C (-38º F).

• If warmer than -39º C (-38º F)

1. Ignition OFF, disconnect the X2 harness connector at the K20 Engine Control Module.
2. Test for infinite resistance between the signal circuit terminal B or 1 and ground.
   • If less than infinite resistance, repair the short to ground on the circuit.
   • If infinite resistance, replace the K20 Engine Control Module.

• Go to next step: If colder than -39º C (-38º F)

5. Install a 3 A fused jumper wire between the signal circuit terminal B or 1 and the low reference circuit terminal A or 2.

6. Verify the scan tool ECT sensor parameter is warmer than 149º C (300º F).

• If 149º C (300º F) or colder

1. Ignition OFF, remove the jumper wire, disconnect the X2 harness connector at the K20 Engine Control Module, ignition ON.
2. Test for less than 1 V between the signal circuit and ground.
   • If 1 V or greater, repair the short to voltage on the circuit.
   • Go to next step: If less than 1 V
3. Ignition OFF.
4. Test for less than 2 Ω in the signal circuit end to end.
   • If 2 Ω or greater, repair the open/high resistance in the circuit.
   • If less than 2 Ω, replace the K20 Engine Control Module.

• Go to next step: If warmer than 149º C (300º F)

7. Test the B34 Engine Coolant Temperature Sensor.

Refer to Component Testing.

Component Testing

1. Ignition OFF, disconnect the harness connector at the B34 Engine Coolant Temperature Sensor.

2. Test the ECT sensor by varying the sensor temperature while monitoring the sensor resistance.

Compare the readings with the Temperature Versus Resistance - Engine Coolant Temperature Sensor table. The resistance values should be in range of the table values.

• If not within the specified range

Replace the B34 Engine Coolant Temperature Sensor.

• Go to next step: If within the specified range

3. Test for infinite resistance between each terminal and the sensor housing.

• If less than infinite resistance

Replace the B34 Engine Coolant Temperature Sensor.

• Go to next step: If infinite resistance

4. Refer to Thermostat Diagnosis.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• Engine Coolant Temperature Sensor Replacement
• Engine Coolant Thermostat Housing Replacement (2.0L LTG) or Engine Coolant Thermostat Housing Replacement (3.6L LGX)
• Control Module References for ECM replacement, setup and programming

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