AutoCodes – GMC – P0128 2018 GMC Sierra

P0128 2018 GMC Sierra Code - Engine Coolant Temperature Below Thermostat Regulating Temperature

Quick Answer

The Engine Control Module (ECM) monitors the temperature of the engine coolant for engine control and as enabling criteria for some diagnostics. Common causes include Leaking or stuck open thermostat, Low engine coolant level, Insufficient warm-up time. Severity is High - diagnose this code as soon as possible. Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.

What it means: P0128 2018 GMC Sierra indicates Engine Coolant Temperature Below Thermostat Regulating Temperature.
Common causes: Common causes include Leaking or stuck open thermostat, Low engine coolant level, Insufficient warm-up time.
Severity: High - diagnose this code as soon as possible.
Can I drive?: Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.

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Code Popularity 488

How popular is this code, among other codes? Low

Repair Importance Level

Urgent (Diagnose now) (3 / 3)

Repair Difficulty Level

Intermediate (2 / 3)

What is this?

Table of contents Created and updated by William Orellana on May 11, 2023

Description
Step-by-Step Diagnostic Guide
Possible Causes
How To Fix
Tech Notes ▶
Cost to Diagnose
FAQs
Comments
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Code P0128 2018 GMC Sierra Description

The Engine Control Module (ECM) monitors the temperature of the engine coolant for engine control and as enabling criteria for some diagnostics. The amount of heat an engine generates is proportional to the amount of air that enters the engine. The ECM monitors this amount of air to calculate a theoretical Engine Coolant Temperature (ECT). The ECM uses the calculated temperature to determine if the engine has warmed up to the thermostat regulating temperature. Suppose the measured actual coolant temperature does not increase typically or does not reach the thermostat regulating temperature. In that case, diagnostics that use the ECT as enabling criteria, may not run when expected. If the measured engine coolant temperature fails to reach the thermostat regulating temperature, before a predetermined amount of air has entered the engine, this Diagnostic Trouble Code (DTC) sets.

OBDII Code P0128 2018 GMC Sierra - Engine Coolant Temperature Below Thermostat Regulating Temperature - AutoCodes.com — P0128 2018 GMC Sierra Code - Engine Coolant Temperature Below Thermostat Regulating Temperature

What does this mean?

What are the Possible Causes of the DTC P0128 2018 GMC Sierra?

Leaking or stuck open thermostat
Low engine coolant level
Insufficient warm-up time
Faulty Engine Coolant Temperature (ECT) sensor
Engine Coolant Temperature sensor harness is open or shorted
Engine Coolant Temperature sensor electrical circuit poor connection

How to Fix the DTC P0128 2018 GMC Sierra?

Review the 'Possible Causes' above and visually examine the corresponding wiring harness and connectors. Check for damaged components and inspect connector pins for signs of being broken, bent, pushed out, or corroded.

What does this mean?

Code Tech Notes

If the cooling system is not leaking and the fluid level is OK, replacing the thermostat usually takes care of the problem.

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What is the Cost to Diagnose the Code?

Labor: 1.0

To diagnose the P0128 2018 GMC Sierra code, it typically requires 1.0 hour of labor. Rates vary by location, vehicle, and shop. Many shops charge between $80–$150 per hour; dealerships and metro areas may be higher, independents may be lower.

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About the author: William Orellana is a Master Mechanic and Mechanical Engineer with over 30 years of experience diagnosing and repairing vehicles. Learn more about his background.

Frequently Asked Questions about P0128 2018 GMC Sierra Code

1. What are the common symptoms of OBDII code P0128 in a 2018 GMC Sierra?

The common symptoms include the check engine light turning on, higher than normal engine temperature readings, and potential issues with the vehicle's heating system.

2. What does the OBDII code P0128 mean in a 2018 GMC Sierra?

OBDII code P0128 indicates that the engine coolant temperature is below the thermostat regulating temperature, suggesting a potential issue with the engine's cooling system.

3. What are the possible causes of OBDII code P0128 in a 2018 GMC Sierra?

Possible causes may include a faulty thermostat, a malfunctioning coolant temperature sensor, low coolant levels, a stuck open thermostat, or issues with the engine's computer system.

4. How serious is OBDII code P0128 in a 2018 GMC Sierra?

While code P0128 may not cause immediate harm to the engine, it can lead to decreased fuel efficiency, increased emissions, and potential engine overheating if left unresolved. It is recommended to address the issue promptly.

5. How can I diagnose OBDII code P0128 in my 2018 GMC Sierra?

Diagnosing code P0128 may involve checking the coolant level, inspecting the thermostat and coolant temperature sensor, and using an OBDII scanner to read live data and confirm the issue.

6. What are the repair options for OBDII code P0128 in a 2018 GMC Sierra?

Repair options may include replacing the thermostat, coolant temperature sensor, or addressing any coolant leaks. It is recommended to consult a professional mechanic for an accurate diagnosis and repair.

7. Can I continue driving my 2018 GMC Sierra with OBDII code P0128?

It is not advisable to ignore OBDII code P0128 and continue driving the vehicle, as it can lead to potential overheating issues and further damage to the engine. It is best to address the problem as soon as possible.

8. How much does it cost to repair OBDII code P0128 in a 2018 GMC Sierra?

The cost of repairing OBDII code P0128 can vary depending on the specific cause of the issue, labor costs, and replacement parts needed. It is recommended to obtain quotes from different mechanics for an accurate estimate.

9. Can a DIY enthusiast fix OBDII code P0128 in a 2018 GMC Sierra?

While some aspects of diagnosing and repairing OBDII code P0128 may be tackled by a DIY enthusiast, it is recommended to seek professional help for an accurate diagnosis and proper repair to ensure the issue is resolved correctly.

10. How can I prevent OBDII code P0128 from occurring in my 2018 GMC Sierra in the future?

To prevent OBDII code P0128, it is important to regularly maintain the vehicle's cooling system by checking coolant levels, replacing the thermostat at recommended intervals, and addressing any cooling system issues promptly. Regular vehicle maintenance can help prevent this code from appearing.