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

Quick Answer

The P0128 diagnostic trouble code (DTC) for a 2018 GMC Sierra indicates that the engine coolant temperature is below the thermostat regulating temperature. Common causes include Faulty Thermostat: The most common cause of the P0128 code is a malfunctioning thermostat that is stuck open, allowing coolant to flow freely and not reaching the desired temperature, Low Coolant Level: Insufficient coolant in the system can prevent the engine from reaching the proper temperature, Coolant Leaks: Leaks in the cooling system can lead to a loss of coolant, affecting the engine's ability to reach the correct temperature. 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 Faulty Thermostat: The most common cause of the P0128 code is a malfunctioning thermostat that is stuck open, allowing coolant to flow freely and not reaching the desired temperature, Low Coolant Level: Insufficient coolant in the system can prevent the engine from reaching the proper temperature, Coolant Leaks: Leaks in the cooling system can lead to a loss of coolant, affecting the engine's ability to reach the correct temperature.
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.
What is this?
Table of contents Created and updated by William Orellana on Sep 14, 2024
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Code P0128 2018 GMC Sierra Description

The P0128 diagnostic trouble code (DTC) for a 2018 GMC Sierra indicates that the engine coolant temperature is below the thermostat regulating temperature. This means that the engine is not reaching the optimal operating temperature as expected. The thermostat in a vehicle's cooling system is designed to regulate the flow of coolant through the engine to maintain a consistent temperature range. When the engine coolant temperature is below the thermostat's set point, it can lead to reduced fuel efficiency, increased emissions, and potential engine damage if not addressed promptly.

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

Common Causes of P0128 2018 GMC Sierra

  1. Faulty Thermostat: The most common cause of the P0128 code is a malfunctioning thermostat that is stuck open, allowing coolant to flow freely and not reaching the desired temperature.
  2. Low Coolant Level: Insufficient coolant in the system can prevent the engine from reaching the proper temperature.
  3. Coolant Leaks: Leaks in the cooling system can lead to a loss of coolant, affecting the engine's ability to reach the correct temperature.
  4. Faulty Coolant Temperature Sensor: A defective coolant temperature sensor can provide inaccurate readings to the engine control module, triggering the P0128 code.
  5. Cooling System Blockage: Blockages in the cooling system, such as a clogged radiator or heater core, can impede the flow of coolant and prevent the engine from reaching the optimal temperature.

Symptoms of P0128 2018 GMC Sierra

  1. Check Engine Light: The most common symptom associated with the P0128 code is the illumination of the check engine light on the vehicle's dashboard.
  2. Poor Fuel Efficiency: A decrease in fuel efficiency may be noticed due to the engine running at a lower temperature than optimal.
  3. Engine Running Cold: The engine may take longer to warm up, and the heater may not produce hot air as quickly as usual.
  4. Rough Idle: The engine may idle roughly or inconsistently due to the lower operating temperature.
  5. Overheating: In some cases, the engine may actually overheat due to the thermostat not functioning properly.

How to Fix P0128 2018 GMC Sierra

  1. Check Coolant Level: Start by checking the coolant level in the reservoir and topping it off if necessary.
  2. Test Thermostat: Test the thermostat to ensure it is functioning correctly. If it is stuck open, replace it with a new one.
  3. Inspect for Leaks: Inspect the cooling system for any leaks and repair them as needed.
  4. Check Coolant Temperature Sensor: Test the coolant temperature sensor to determine if it is providing accurate readings. Replace it if necessary.
  5. Flush Cooling System: If there are blockages in the cooling system, consider flushing the system to remove any debris and improve coolant flow.

Cost to Fix P0128 2018 GMC Sierra

The cost of repairing a P0128 code on a 2018 GMC Sierra can vary depending on the specific cause of the issue. On average, the cost of replacing a thermostat ranges from $150 to $300, including parts and labor. If additional repairs are needed, such as fixing coolant leaks or replacing a faulty sensor, the total cost could increase. It's important to note that diagnostic time and labor rates at auto repair shops can vary, so it's advisable to check with local shops for a more accurate estimate. Keep in mind that rates may be higher at dealerships or in metropolitan areas, while independent shops may offer more competitive prices.

What does this mean?

Tech Notes for P0128 2018 GMC Sierra

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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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.

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