Purpose

This article outlines the procedures to diagnose and confirm Diagnostic Trouble Code (DTC) P0128 on a 2017 GMC Terrain and to replace the Thermostat to resolve the issue.

Introduction

In cases where the Malfunction Indicator Lamp (MIL) on your vehicle’s dashboard is illuminated and the Engine Control Module (ECM) has stored DTC P0128 (Engine Coolant Temperature (ECT) Below Thermostat Regulating Temperature), it indicates the ECT fails to reach the thermostat regulating temperature.

A thermostat is a component of the engine cooling system. The primary function of the cooling system is to regulate the temperature of the engine, and maintain it at an optimal operating temperature. This ensures efficient performance and prevents overheating. Operating a vehicle with a malfunctioning thermostat has several potential consequences, ranging from decreased fuel efficiency to severe engine damage.

To correct this DTC, inspect the vehicle’s cooling system and ECT Sensor, and replace the thermostat as needed to correct this problem.

Symptoms of DTC P0128 include:

• Malfunction Indicator Lamp (MIL) ON or flashing
• Malfunction Indicator Lamp (MIL) ON or flashing
• Poor fuel economy

Possible causes of this fault include:

• Faulty thermostat
• Faulty ECT Sensor

On a 2017 GMC Terrain, replacing the thermostat may resolve DTC P0128.

	Step Time required (hour) Sections Cautions	2 1.4 5 0

(This image is for illustrative purposes only and may not match the actual part number)

Inspection and Diagnosis:

Initial Inspection

During your initial inspection, check for obvious mechanical signs such as torn inlet boots, missing or broken vacuum tees, and/or cracked, broken, or disconnected wire harnesses. Make sure all harness connectors are properly placed in the connectors and holders. This will prevent potential open or short-circuit issues. Also, check the connector for corrosion, frayed wiring, and damaged terminals. In addition, inspect the battery terminals and check the state of charge before attempting to diagnose possible electrical faults.

Inspect the Engine Coolant Temperature Sensor (ECT) to ensure the sensor fits tightly on the block engine. Check the properly filled cooling system, proper pressure maintained by the radiator cap, proper antifreeze–water mixture, and check the cooling fan for operating correctly; if not, the engine may overheat. Refer to the Vehicle Emission Warranty Manual for time and mileage coverage for the emission-related fault(s). For additional information, refer to the Service Manual or applicable Technical Service Bulletin (TSB).

Diagnostic Procedures

1. Ignition ON, verify that other DTCs are not set.

2. Verify the cooling level is corrected. If not, fill the coolant to the correct level.

3. Run the engine with the A/C OFF within 15 minutes, after that raise the engine speed to 3,000 RPM.

4. Use the Scan Tool to verify the Engine Coolant Sensor parameter is greater than 80⁰C (176⁰F).

If not, replace the Engine Coolant Thermostat.

Replace the Thermostat

Removal

Step 1 – Use a lift to raise the vehicle.

Step 2 – Drain the cooling system.

Step 3 – Disconnect the coolant outlet hose from the engine, and the radiator surge tank outlet hose from the thermostat housing.

Step 4 – Loosen the bolts on the thermostat housing.

Install

Installation is accomplished in the reverse order of removal.

Validate the Repair:

1. Erase DTC(s) and Freeze Frame (FF) data.

2. Perform the Key ON Engine OFF (KOEO) and Key ON Engine RUNNING (KOER) test or perform the specific drive cycle requirements for the fault area. Optionally, operate the vehicle within the conditions recorded in the Freeze Frame data when DTC P0128 was initially set.

3. Check for pending or stored DTCs. If none are found, the repair is complete.