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P0128 - How To Repair: Meaning, Causes, Symptoms, Fixes

P0128 - OBD2 Trouble Code Coolant Thermostat

P0128 Code Definition

Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

What Does the Engine Code P0128 Mean?

The character "P" in the first position of diagnostic trouble code (DTC) represents the powertrain system (engine and transmission), "0" in the second position means that this is a generic OBD-II (OBD2) DTC. The "1" in the third character position in a DTC indicates that the fuel and air metering system is experiencing a malfunction. The last two characters "28" are DTC numbers. OBD2 Diagnostic Trouble Code P0128 indicates that coolant temperature is below thermostat regulating temperature. When the engine first starts up, the engine coolant temperature (ECT) sensor and an intake air temperature (IAT) sensor both have similar temperature readings. Because the engine is running, the powertrain control module (PCM) would expect the ECT sensor reading to increase to normal operating temperature within a few minutes at idle, or sooner when driving. If the ECT reading is the same as the IAT reading or only slightly higher than the IAT reading, PCM would generate and store DTC P0128 in the fault memory. The P0128 trouble code means that the engine temperature was not achieved warm enough or fast enough when the engine was started. The code P0128 also often indicates a malfunctioning thermostat.

Carly's expert mechanics have identified the following related fault codes. With the information for these fault codes it is often possible to identify and better repair your faulty part/system:


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    What Is the Cause of P0128?

    While in most cases the code P0128 is related to a defect or permanently open thermostat, P0128 can also be caused by other issues:

      A failed engine coolant temperature (ECT) sensor

      Faulty wiring connecting the ECT sensor

      A low coolant level

      Another problem with the coolant system

    What Are the Symptoms of Code P0128?

    The main symptom to the driver is the illumination of the check engine light (CEL), also called malfunction indicator lamp (MIL). In most cases, the driver will not notice any symptoms in drivability, although there may be the following symptoms:

      Longer engine warm-up time

      The heater might not get hot enough

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    How much does it cost to diagnose P0128?

    Visiting a car repair shop and letting them perform a diagnostics check can cost you $60-$100. For almost the same price you can get Carly. Its advanced features can cover what a professional automotive scanner that costs over $3,000 would, and more!

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    Which Tools Are Needed To Diagnose P0128?

    To find out more about the P0128 code that is generated and stored in the Powertrain Control Module (PCM) of your car, the only tool you need is the Carly Universal OBD2 Scanner, which works for all car brands in the world with an OBD2 port. Carly is a revolutionary solution that allows you to access your car’s data with your phone and translates this information into powerful insights, predictions, and possibilities. This saves you trouble, time, and money.

    How To Diagnose and Clear the Code P0128?

    Performing a diagnostics check with a diagnostic scanner helps you identify a particular problem area that is outside the normal range and shows where a fault might be occurring. This helps you to detect errors before they cause more damage to your vehicle. In comparison to generic OBD devices that only provide OBD-level engine-related data, Carly can run advanced manufacturer-level diagnostics, check all your ECUs in your car (like ABS, airbag, multimedia) and get deeper insights. You can find additional information about the trouble code and understand the severity of the problem. With the new Smart Mechanic feature, you get additional professional insights and tips from expert mechanics. You are able to check components and their mechanics, find detailed repair tips, and learn about potential future damage. After you have fixed the problem you can easily clear the fault codes with Carly, too. Also, the diagnostic results are safely stored inside the app or as convenient PDF reports you can share with your friends or a mechanic.

    Common Mistakes When Diagnosing the P0128 Code

    The most common mistake is to look and try to fix each individual OBD2 code without having a holistic approach. Understanding the interrelationship of OBD-II (OBD2) systems is crucial to determining the root cause of the problem and finding the right solution. For example, the most common cause of the P0128 code is a faulty thermostat or one that is stuck open. However, while it is the most common cause, this is not always the reason. Many people having the trouble code P0128 rush to replace the thermostat without properly checking the car. While in most cases, it may be the right solution, there are situations where it will not solve the real issue.

    How Serious Is Code P0128?

    While it is common for a driver to see no symptoms impacting driveability when the trouble code P0128 is triggered, there is a small chance that driving with this code can damage your engine. To avoid costs of potential engine damages and potential increase of harmful emissions, it is recommended to have this code properly diagnosed and fixed as soon as possible.

    How to Fix the P0128 Code

    Depending on the root cause of the problem, one or more of the following repairs can fix the P0128 code:

      Inspect and if needed replace the thermostat

      Inspect the engine coolant temperature (ECT) sensor

      Check the connector and wiring of ECT sensor

      Inspect the intake air temperature (IAT) sensor

      Inspect the cooling fan

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    Estimated Cost of Repair for the P0128 Code

    Depending on the root cause of the issue, the costs of fixing the code P0128 will most likely range from $150-$250.

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    Learn More about the P0128 Code

    What Is an Engine Coolant Temperature (CTS, ECTS, ECT) Sensor?

    The engine coolant temperature (ECT) sensor (also abbreviated ECTS), also called the coolant temperature sensor (CTS), is typically a negative temperature coefficient (NTC) thermistor that correlates the temperature of the liquid coolant with the magnitude of resistance by decreasing resistance as the temperature increases. The powertrain control module (PCM) sends to the ECT sensor a reference voltage and constantly monitors the returning signals of the ECT sensor.

    Where Is the Engine Coolant Temperature (ECT) Sensor Located?

    The engine coolant temperature (ECT) sensor is usually mounted near the thermostat in the engine block or cylinder head.

    *Every car is unique. The Carly Features that your car supports depend on the model, build year, hardware, and software in the car. Plug the scanner in the OBD2 port, connect to the app, run your first diagnostics, and check which features are available for your car. Also, keep in mind that any information provided on this website is for informational purposes only and should be used at your own risk. Mycarly.com is not responsible for any errors or omissions, or for the results stemming from the use of this information.

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