P0128 Subaru | The Expert’s Guide

The P0128 Subaru error code in vehicles is a common issue that can cause concern for owners. This code is related to the engine coolant temperature being below the thermostat regulating temperature.

In this article, we will explore the potential causes, symptoms, and possible solutions for the P0128 error code in Subaru vehicles.

Symptoms of P0128 Subaru Error Code

When the P0128 error code is triggered in a Subaru vehicle, there are several symptoms that may indicate a problem:

Check Engine Light (CEL) Illumination

The most common symptom associated with the P0128 error code is the illumination of the check engine light on the dashboard. This light serves as a warning and indicates that the vehicle’s onboard diagnostics system has detected a potential issue.

Engine Performance Issues

In some cases, you may experience a decrease in engine performance, such as reduced power or rough idling. This can be attributed to the improper functioning of the coolant temperature regulation, affecting the engine’s overall efficiency.

Temperature Gauge Reading

The temperature gauge on the dashboard may stay on the cold side or not rise to the normal operating temperature.

This indicates that the engine is not reaching its optimal operating temperature, which can lead to poor fuel efficiency and potential engine damage over time.

Causes of P0128 Error Code

The P0128 error code can be triggered by various issues within the Subaru vehicle. Some of the potential causes include:

Faulty Thermostat

A malfunctioning or stuck-open thermostat can cause the coolant temperature to remain below the desired operating range, triggering the P0128 error code.

The thermostat is responsible for regulating the flow of coolant through the engine, and if it fails to open or close properly, it can disrupt the engine’s temperature balance.

Low Coolant Level

Insufficient coolant in the system can disrupt the proper functioning of the thermostat, leading to the P0128 error code.

Coolant is crucial for maintaining the engine’s temperature within the desired range, and a low coolant level can prevent the thermostat from functioning optimally.

Coolant Sensor Issues

A faulty coolant temperature sensor may provide incorrect readings to the engine control unit (ECU), resulting in the error code. The coolant temperature sensor plays a vital role in providing accurate temperature data to the ECU, which then adjusts various engine parameters accordingly.

If the sensor is malfunctioning, it can lead to incorrect temperature readings and trigger the P0128 error code.

Faulty ECU

In rare cases, a malfunctioning ECU can incorrectly interpret temperature data and trigger the P0128 error code. The ECU is responsible for monitoring and controlling various aspects of the engine’s operation, including coolant temperature regulation.

If the ECU is not functioning correctly, it can lead to erroneous error code triggers.

Wiring and Connection Problems 

Loose or damaged wiring and connections between the coolant temperature sensor, thermostat, and ECU can cause issues, leading to the error code.

Proper electrical connections are crucial for accurate temperature readings and communication between these components. Any damage or disruption to the wiring and connections can result in the P0128 error code.

Diagnosing the P0128 Error Code

Understanding the shared challenges across different vehicles gives you a broader perspective, potentially simplifying the resolution process for the P0128 code.

To diagnose the P0128 error code in a Subaru vehicle, it is recommended to follow these steps:

Check Coolant Level

Ensure that the coolant level is within the recommended range. If it is low, top it up and monitor for any leaks. A low coolant level can disrupt the thermostat’s operation and trigger the error code.

Inspect Thermostat

Test the thermostat to ensure it is opening and closing properly. If it appears faulty, consider replacing it. A malfunctioning thermostat can cause the coolant temperature to remain below the desired operating range, resulting in the error code.

Check Coolant Temperature Sensor

Inspect the coolant temperature sensor for any signs of damage or corrosion. If necessary, replace the sensor and clear the error code.

A faulty sensor can provide incorrect temperature readings to the ECU, leading to the error code trigger.

Inspect Wiring and Connections

Thoroughly examine the wiring and connections related to the coolant temperature sensor, thermostat, and ECU. Repair or replace any damaged components as needed.

Proper electrical connections are crucial for accurate temperature readings and communication between these components.

Reset Error Code

After performing the necessary repairs or replacements, use an OBD-II scanner to clear the error code and reset the check engine light. This will ensure that the system recognizes the corrective actions taken and no longer triggers the error code.

Fixing the P0128 Code

Once you have diagnosed the cause of the P0128 code, it is time to address the issue appropriately. Here are some common solutions for fixing the P0128 code in a Subaru:

Replacing the ECT Sensor

If the coolant temperature sensor is faulty, replacing it with a new one should resolve the issue. It is important to use a genuine Subaru replacement part for optimal performance and compatibility.

Replacing the Thermostat

If the thermostat is stuck open or malfunctioning, replacing it can restore the proper coolant flow and allow the engine to reach the desired operating temperature more efficiently. Ensure that the new thermostat is compatible with your Subaru model.

Repairing Cooling System Issues

Address any identified cooling system issues promptly. Whether it’s a radiator problem, a malfunctioning water pump, or faulty cooling fans, repairing or replacing the defective components will help restore the cooling system’s functionality.

It is essential to note that diagnosing and fixing the P0128 code can vary depending on your specific Subaru model and its unique characteristics.

Consulting a professional mechanic or referring to your vehicle’s service manual can provide more detailed instructions tailored to your situation.

Conclusion

The P0128 error code in Subaru vehicles can be a cause for concern, as it indicates an issue with the engine coolant temperature regulation.

By understanding the potential causes, symptoms, and diagnostic steps, you can effectively address this problem. Remember to perform regular maintenance, address issues promptly, and use quality coolant to prevent the recurrence of the P0128 error code in your Subaru vehicle.

Taking these preventive measures will help ensure the longevity and reliability of your vehicle’s cooling system.

FAQ

1. What are the symptoms of the P0128 error code in Subaru vehicles?

• Check Engine Light (CEL) Illumination
• Engine Performance Issues
• Temperature Gauge Reading

2. What are the potential causes of the P0128 error code?

• Faulty Thermostat
• Low Coolant Level
• Coolant Sensor Issues
• Faulty ECU
• Wiring and Connection Problems

3. How can I diagnose the P0128 error code in my Subaru vehicle?

• Check Coolant Level
• Inspect Thermostat
• Check Coolant Temperature Sensor
• Inspect Wiring and Connections
• Reset Error Code

4. What preventive measures can I take to avoid encountering the P0128 error code in the future?

• Regular Maintenance
• Use Quality Coolant
• Monitor Coolant Level
• Address Issues Promptly