AutoCodes – GMC – P0128 2015 GMC Sierra

P0128 2015 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 2015 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 397

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 2015 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 2015 GMC Sierra - Engine Coolant Temperature Below Thermostat Regulating Temperature - AutoCodes.com — P0128 2015 GMC Sierra Code - Engine Coolant Temperature Below Thermostat Regulating Temperature

What does this mean?

What are the Possible Causes of the DTC P0128 2015 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 2015 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 2015 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 2015 GMC Sierra Code

What does the OBDII code P0128 mean for a 2015 GMC Sierra?

P0128 indicates that the engine coolant temperature is not reaching the expected level for proper operation, suggesting an issue with the thermostat or cooling system.

What are the common symptoms of code P0128 in a 2015 GMC Sierra?

Symptoms may include a check engine light, poor fuel efficiency, the engine taking longer to warm up, or reduced heater performance.

What causes the P0128 code to appear on a 2015 GMC Sierra?

Common causes include a stuck-open thermostat, low coolant levels, a faulty coolant temperature sensor, or issues with the engine control module (ECM).

Can a P0128 code damage my 2015 GMC Sierra engine?

While the code itself isnâ€™t immediately damaging, prolonged driving with an unresolved P0128 can cause increased wear on engine components and reduced fuel efficiency.

How can I diagnose a P0128 code on my 2015 GMC Sierra?

Diagnosis involves checking the thermostat operation, inspecting the coolant level and condition, testing the coolant temperature sensor, and reviewing live temperature data with an OBDII scanner.

Can I still drive my 2015 GMC Sierra with a P0128 code?

Yes, but itâ€™s not recommended for long periods, as it can lead to poor fuel economy and potentially strain engine components over time.

How do I fix the P0128 code on a 2015 GMC Sierra?

Common fixes include replacing the thermostat, refilling or flushing the coolant, replacing a faulty coolant temperature sensor, or addressing wiring issues.

How much does it cost to repair a P0128 code on a 2015 GMC Sierra?

Repair costs vary, but replacing a thermostat typically ranges from $150 to $300, depending on labor rates and parts pricing.

Can a faulty thermostat trigger the P0128 code on a 2015 GMC Sierra?

Yes, a thermostat stuck in the open position is one of the most common causes of the P0128 code.

How often should I check the coolant system to prevent a P0128 code on my 2015 GMC Sierra?

Itâ€™s a good idea to inspect the coolant level, condition, and thermostat operation during regular maintenance, such as oil changes or at least once a year.