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P0125 Code – What Does It Mean & How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to fix it, and what other codes may show related to it.


DTC Code P0125: Insufficient coolant temperature for closed-loop fuel control


For every vehicle, there is a specific temperature the engine is expected to reach for its optimum overall performance. An engine temperature lower than the manufacturer-defined temperature requirement will require the engine to burn more fuel, thereby increasing fuel consumption. On the other hand, if the temperature is too high, it may result in severe damage to the engine.

In order to maintain the normal engine temperature, the engine coolant temperature (ECT) sensor is designed to measure the engine temperature after the engine starts.

DTC code P0125 is registered when the Engine Control Module (ECM) is unable to record a coolant temperature equal to the required engine temperature. This code implies that the engine temperature has not reached the threshold temperature levels required to enter closed-loop operation within a specific time-lapse.


There are a number of factors or malfunctions that could trigger a vehicle to register the DTC Code P0125. However, the below-listed faults have been notorious for the occurrence of P0125, among other OBD-II codes in a vehicle. The OBD-II generic code will most likely be registered when one or more of the following troubles occur in the vehicle:

  • Insufficient engine warm-up time
  • Engine Coolant Temperature (ECT) is defective or damaged
  • The Coolant Temperature sensor is defective
  • The wiring connecting the ECT sensor to the engine control module (ECM) is faulty or dysfunctional
  • ECT sensor connector is corroded
  • The plug connecting to the ECT sensor is disconnected
  • Inability of the Engine Coolant thermostat to open at the proper temperature
  • The engine has a low coolant level
  • Possible coolant leaks in the system
  • Leaks in the open thermostat
  • Engine cooling fan is defective
  • Damaged engine control module. This may, however, prompt OBD-II trouble codes other than P0125.


Although DTC code P0125 does not usually have a serious effect on the vehicle’s drivability, it, however, reduces fuel economy among other side effects. Here are some of the symptoms that indicate the presence of DTC code P0125 in a vehicle:

  • Illumination of the check engine light
  • High fuel consumption/low fuel economy
  • Excessive heat generated by the engine
  • Vehicle overheating
  • Reduced or poor heater performance


It is generally believed that the occurrence of DTC code P0125 is a result of a faulty thermostat, this is, however, not always the case. Therefore, diagnosing this trouble code should be carefully done to be certain of the root cause of the problem. Meanwhile, the following diagnostic processes have been designed to serve as a careful guide to easily detect the cause and clear the vehicle of the P0125 error.

To start with, the trouble code is triggered when the ECM is unable to detect an engine temperature that equals the vehicle’s predefined operational temperature. Therefore, diagnosing DTC code P0125 should start by first taking the engine’s temperature reading. To detect the actual cause of this trouble code, compare your reading with the reading on the scan tool. To do that, follow the instructions below:

  • To begin the diagnostic process, you need an advanced scan tool and an infrared thermometer capable of sensor reading
  • Equipped with these tools, proceed to take the data reading on the scan tool and compare it with the temperature reading. While at it, ensure that the two temperature readings match
  • Then, proceed to visually inspect the coolant level when the engine is cold. If a low coolant level is observed, refill the coolant and check for possible leaks in the system
  • Having completed the steps above, clear the code and take the vehicle for a test drive to see if the code returns
  • If the code returns, use an advanced scan tool to read data from the engine control module (ECM)
  • Analyze the ECM data to be sure if the thermostat is responsible for this error
  • Subsequently, visually inspect the thermostat to ensure it opens correctly
  • If certain that the error is a result of a faulty thermostat, replace the thermostat
  • Finally, clear the error and test the vehicle to see if the error returns.

Banking on the observation after performing the test drive as instructed above, you may consider the following additional diagnostic processes:

  • Bleed the cooling system properly to remove air compartments, by so doing, preventing the engine from overheating
  • Visually inspect the cables and connectors in the ECM sensor and ensure they are in good condition. Repair any defects detected
  • Visually inspect the Engine Coolant Temperature (ECT), repair or replace the ECT as may be required

Common mistakes

Having stated that DTC code P0125 is mostly caused by a faulty thermostat, it is, however, not in all cases. Therefore, it would be considered a mistake to hastily replace the thermostat without being sure that the error is caused by a defective thermostat. At the same time, it will reduce the cost of repairing the vehicle. More so, overheating is equally a major cause, therefore the mechanic is expected to bleed the cooling system properly to prevent the vehicle from overheating. 

Another equally common mistake is the failure to conduct a visual inspection on the engine coolant system and testing the vehicle with an advanced scan tool. This is expected to be done in order to detect the actual cause of the DTC code P0125.

How Serious is P0125?

While DTC code P0125 does not usually affect the vehicle’s drivability or prevent the vehicle from running, there are, however other serious effects of this trouble code that will prompt you to repair the vehicle with immediate effect. These other effects of DTC code P0125 may include:

  • Overheating of the vehicle
  • Blockage of the heat coming out of the vents
  • Reduced fuel economy
  • Fuel instability, which may cause damage to the engine
  • Emissions passing may be prevented


What Repair Can Fix P0125?

In addition to the diagnostic processes mentioned above, you may consider the following repairs to effectively clear the vehicle of this OBD-II trouble code:

  • Repair or replace the engine coolant temperature (ECT) sensor as appropriate
  • Replace the thermostat if it is responsible for the occurrence of the trouble code
  • Check the wirings and connections in the ECM sensor and ensure they are well connected
  • Check for possible leaks in the engine coolant system and repair or replace as appropriate
  • Replace the engine cooling fan if damaged
  • If the ECT sensor is corroded, clean the sensor or replace it if damaged beyond what cleaning can solve



Regardless of the fact that DTC code P0125 will not prevent your vehicle from running, the other damages it causes to the engine are enough to hurry you to repair the vehicle as soon as the error is detected. Finally, before replacing any parts, especially the thermostat, be sure that it is responsible for the occurrence of this trouble code.

P0125 Code – What Does It Mean & How To Fix It



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