Description of the P0128 fault code.
OBD2 Error Codes

P0128 Coolant thermostat malfunction

P0128 – OBD-II Trouble Code Technical Description

Trouble code P0128 indicates that the coolant temperature is below the thermostat opening temperature.

What does the fault code mean P0128?

Trouble code P0128 indicates problems with engine coolant temperature. This usually means that the engine is not reaching the required heating temperature within the specified time frame.

Coolant thermostat.

Possible reasons

Some possible reasons for the P0128 trouble code:

  • Faulty Thermostat: A faulty thermostat may not open or close correctly, resulting in under- or over-temperature coolant.
  • Low coolant level: Insufficient coolant level can result in insufficient engine cooling and consequently low temperatures.
  • Faulty Temperature Sensor: A faulty engine temperature sensor can cause the coolant temperature to be incorrectly read.
  • Faulty Cooling System: Problems with the coolant pump or other cooling system components can cause the engine to not cool properly.
  • Faulty Air Temperature Sensor: If the intake manifold air temperature sensor is faulty, it may affect the performance of the cooling system.
  • Wiring or Connection Problems: Faulty wires or connections can cause sensor signals to not transmit correctly, which can cause P0128.
  • Malfunctioning Engine Control Module (ECM): In rare cases, problems with the Engine Control Module itself can result in a P0128 code.

What are the symptoms of a fault code? P0128?

Some possible symptoms for trouble code P0128:

  • Increased engine warm-up time: The engine may take longer than usual to warm up to optimal operating temperature.
  • Low Coolant Temperature: When reading the coolant temperature sensor, the instrument panel or scan tool may display a low temperature even though the engine should have already warmed up.
  • Increased fuel consumption: Due to insufficient engine temperature, the fuel management system may enter a rich mixture mode, which can lead to increased fuel consumption.
  • Poor Engine Performance: Inadequate engine cooling can affect overall engine performance, which may result in loss of power, vibration, or other operating abnormalities.
  • Limp Start: In some cases, the ECM may put the engine into a limp mode to prevent damage due to insufficient cooling temperature.

How to diagnose a fault code P0128?

The following steps are recommended to diagnose DTC P0128:

  1. Check Coolant Temperature (ECT) Sensor:
    • Check the electrical connections of the ECT sensor for corrosion, oxidation, or breaks.
    • Use a multimeter to test the sensor resistance at different temperatures. The resistance should change according to the temperature change.
    • Check for coolant leaks where the ECT sensor is located.
  2. Check the thermostat:
    • Make sure the thermostat is working properly, opening and closing when it reaches a certain temperature.
    • Check to see if the thermostat is stuck in the closed or open position.
  3. Check the cooling system:
    • Check the level and condition of the coolant. Leaks or insufficient coolant can result in insufficient engine cooling.
    • Check the operation of the cooling fan. Make sure it turns on when it reaches a certain temperature.
  4. Check Engine Control Module (ECM):
    • Use a scan tool to read other error codes and check sensor and actuator data associated with the cooling system.
    • Check the ECM software for updates or errors.
  5. Check wiring and connections:
    • Check the wiring from the ECT sensor to the ECM for breaks, corrosion, or breaks.
    • Check connections and clamps for oxidation or distortion.

After diagnostics have been carried out and the problem has been identified, the necessary repairs should be made or defective components should be replaced.

Diagnostic errors

When diagnosing DTC P0128, the following errors are possible:

  • Incorrect interpretation of coolant temperature (ECT) sensor data:
    • Incorrect reading of the ECT sensor may lead to misinterpretation of the cause of the problem. It is important to correctly interpret temperature readings to determine if the engine is heating up too quickly or too slowly.
  • Ignoring other potential problems in the cooling system:
    • Code P0128 can be caused not only by insufficient engine cooling, but also by other problems such as a malfunctioning thermostat or coolant leaks. Ignoring these potential problems may lead to misdiagnosis.
  • Not conducting a complete diagnosis:
    • Failure to fully diagnose the cooling system, including checking the temperature sensor, thermostat, coolant condition, and cooling fan operation, may result in missing the true cause of the error.
  • Incorrect interpretation of scanning error code:
    • The P0128 error code does not always indicate a specific problem. It is important to analyze the scan data along with other symptoms and diagnostic results to accurately determine the cause of the problem.
  • Wrong solution to the problem:
    • Failure to correctly identify and correct the problem can result in lengthy repair times and additional costs. It is important to conduct a full diagnosis and contact professionals if necessary.

How serious is the fault code? P0128?

Trouble code P0128 indicates potential problems with the engine cooling system. While it can be caused by a variety of factors, including a malfunctioning thermostat or temperature sensor, insufficient engine cooling can lead to overheating, engine damage, and even engine failure. Therefore, the P0128 code should be taken seriously and it is recommended to immediately diagnose and repair the problem to prevent serious engine damage.

What repair will help eliminate the code? P0128?

Troubleshooting DTC P0128 may include the following:

  • Replacing Thermostat: If the thermostat is not functioning properly, the engine may not heat up enough, resulting in a P0128 code. Thermostat replacement may be necessary.
  • Checking the Coolant Temperature Sensor: If the temperature sensor is not producing the correct signals, this can also cause the P0128 code. Check it for proper operation and replace if necessary.
  • Cooling System Check: Check the cooling system for leaks, insufficient coolant, or other problems that could cause the engine to overheat.
  • Checking the Cooling Fan Operation: If the cooling fan is not functioning properly, it can also cause the engine to overheat. Make sure the fan runs when a certain temperature is reached.
  • Check Wiring and Connections: Check electrical wiring and connections to ensure there are no breaks or corrosion that could cause the sensors to malfunction.

The repair will depend on the specific cause of the P0128 code in your specific vehicle. If you are not experienced in auto repair, it is recommended that you contact a qualified auto mechanic to diagnose and repair the problem.

How to Fix P0128 Engine Code in 3 Minutes [2 DIY Methods / Only $7.34]

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