how to diagnose P0128 code in GMC Sierra T1XX (2019-Present)

Learn how to diagnose and fix P0128 code in GMC Sierra T1XX. Step-by-step guide with diagnostic procedures and repair instructions for cooling system issues.

Solution

1. Preparation

• Tools Required: OBD-II scanner, multimeter, socket set, torque wrench, coolant.
• Allow the engine to cool completely before starting work to prevent burns.

2. Replace the Thermostat

1. Disconnect the battery to ensure safety while working on electrical components.
2. Drain coolant by removing the drain plug from the radiator.
3. Remove any components obstructing access to the thermostat (e.g., air intake duct).
4. Unbolt the thermostat housing using a socket wrench and remove the housing.
5. Take out the old thermostat and inspect it for proper operation.
6. Clean the mating surfaces on the engine and housing to ensure a proper seal.
7. Install the new thermostat in the correct orientation.
8. Reattach the thermostat housing and tighten bolts to the manufacturer’s torque specifications.
9. Reconnect the battery.

3. Replace the Coolant Temperature Sensor (if needed)

1. Locate the coolant temperature sensor on the engine (typically near the thermostat).
2. Disconnect the electrical connector from the sensor.
3. Remove the sensor using a socket or wrench.
4. Install the new coolant temperature sensor and tighten to the manufacturer’s specification.
5. Reconnect the electrical connector.

4. Refill and Bleed Coolant System

1. Refill the cooling system with the appropriate coolant type.
2. Bleed the cooling system to remove any air pockets, ensuring efficient cooling system operation.
3. Start the engine and let it reach operating temperature while monitoring the coolant level.