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DTC P0128

15.06.2024
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P0128
Vehicle engine and/or automatic transmission
Code: P0128 - Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

Essentially, the fault code P0128 means that the engine coolant is not warming up quickly enough. This indicates that there is an issue with the engine not reaching its operating temperature within a specified time. The Engine Control Module (ECM) determines and controls the time required for this, based on the ambient temperature inside the car.

Automobile brands like Renault, Toyota, KIA, Škoda, Nissan, Hyundai, Volkswagen, Mercedes, BMW, Ford, and others may display the mentioned OBD code.

OBD P0128 Causes

There are several common issues that can trigger the P0128 fault code:

  • The most common cause is that the coolant thermostat is stuck open or operates prematurely.
  • The second most likely cause is a faulty or damaged coolant temperature sensor or the wiring associated with the sensor.

Symptoms of Error P0128

There are two common signs that occur when a car has the P0128 fault code:

  • The heater does not warm up enough due to the low coolant temperature in the engine.
  • The car takes too long to warm up, or you notice that the temperature gauge does not rise as high as it used to.

If you notice any of these signs during a check or test drive, it could be confirmation of the P0128 fault code.

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How to Diagnose DTC P0128?

There are several logical steps a mechanic must take to properly diagnose and confirm the P0128 fault code. Keep in mind that diagnostic methods and procedures may vary, but in general, there is a set of steps each mechanic should follow to assess this particular fault code:

  • The first thing to do is check the coolant temperature as it begins to flow through the radiator hose to the thermostat.
  • Generally, the hose should be mildly warm until the thermostat opens.
  • When the thermostat opens, very hot coolant starts flowing and quickly heats up the radiator hose. You should be cautious as it is very hot and could cause burns.
  • If you notice that the radiator hose heats up slowly and does not become hot quickly or suddenly, it indicates that the thermostat is stuck open. If the hose heats up quickly as it should, the next step is to check the readings from the coolant temperature sensor.
  • To ensure your measurement is accurate, use an OBD-II scanner, but you can also use the temperature gauge in your vehicle. Compare the readings obtained from the infrared thermometer. If everything is working correctly, the thermometer should show around 200°F / 94°C. If the scanner’s reading does not match the thermometer's measurement (or is at least close), there is an issue with the sensor inside the car.
  • If the sensor is the issue, connectors usually begin to degrade. A poor connection can be the source of this problem and lead to further issues with the vehicle. If the connection looks proper and works, then the problem could be with the sensor itself.
  • In rare cases of the P0128 fault code, there may be an issue with the voltage supply provided by the ECM. This is hard to address in detail as you would need factory-level information specific to your vehicle, which can vary from car to car. You need knowledge in electronic diagnostics to avoid damaging the vehicle.
dtc p0128

Common Mistakes When Dealing with OBD Code P0128

The most common mistake made when addressing the P0128 fault code issue is replacing the thermostat without properly checking other parts of the car.

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How Serious Is the P0128 Fault?

This is a relatively minor fault code, but remember that if the vehicle is used in this state for a long time, it can lead to more significant problems. There is a small possibility that contamination of water in the oil can lead to its degradation and engine damage.

With this code, caused by the engine coolant not warming up enough, it means the engine doesn't get hot enough to evaporate condensation. If that's the case, water can enter the oil. Even though this particular condensation issue may not cause noticeable damage for several years, there is still a chance of damaging your engine.

What Repairs Can Fix the P0128 Fault Code?

The simplest repairs to perform to resolve the code include:

  • Replacing the thermostat. This is the most common repair.
  • Replacing the coolant temperature sensor, wiring, and/or connector associated with the sensor and damaged.

Conclusion

If you believe you have the P0128 fault code, it should be assessed and confirmed as soon as possible. Even though the likelihood of this causing serious damage to your vehicle is low, there is still a chance.

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OBD P0128 code analysis and solutions

The page presents a in-depth analysis of code P0128 for Opel, Renault, VW (Volkswagen), Ford, Peugeot, Mitsubishi, Skoda, BMW, Citroën, Toyota, Lada, Honda, Kia, and other models to analyze probable causes and solutions related to obd-ii errors, with focus on the issue. We offer technical documentation and methods for diagnostics to identify and resolve the issue effectively. Our resource is designed to be a comprehensive source of information on OBD 2 errors, providing you with the necessary tools for successful repair and maintenance of your automobile. If during the fixing of OBD-2 errors you need to purchase parts, don’t forget to visit Avtopro!