What does the DTC P0115 mean?
By
Vroomouk
This blog will explain the meaning of the fault code P0115 and answer the following questions: what are the causes and remedies for the P0115 fault code? How to diagnose a P0115 fault code? Can you drive with a DTC P0115?
What does the DTC P0115 mean?
The DTC or diagnostic trouble code P0115 means the coolant temperature sensor of the engine has malfunctioned. The signal from the coolant circuit should be in the range of -20 to 120+ degrees celsius. When the control unit receives an implausible signal it uses a default value of -40 degrees celsius and continues to operate the engine fan in limp home mode.
The coolant temperature sensor is the most important sensor in the engine. It is located on the cylinder head and the engine control unit is directly connected to the coolant temperature sensor.
All the coolant temperature sensors used in internal combustion engines are NTC or negative temperature coefficients. The internal resistance of the sensor reduces as the temperature increases.
Car manufacturers while programming the engine control unit define a temperature for the corresponding resistance value. The engine control unit then controls sub-functions like the warm-up phase or overhead protection mode based on the resistance of the coolant temperature sensor.
When the coolant temperature is low, the reference voltage from the engine control unit encounters maximum resistance from the coolant sensor. The reference voltage can be 12 V or 5 V depending on the manufacturer of the engine control unit.
As the engine reaches its operating temperature, the resistance of the coolant temperature sensor goes down because the probe is in contact with the coolant. The engine control unit can then maintain the operating temperature by actuating the thermostat and engine cooling fan.
What are the causes and remedies for the P0115 fault code?
The causes and their remedies for the P0115 fault code are explained below
- open/ short circuits in the electrical lines
The coolant temperature sensor is connected to the engine control unit via two electrical lines. One line is a positive reference voltage and the signal line is the negative or the ground. Both lines go back into the ECU and the control unit internally grounds the complete circuit.
Therefore, if there is an open circuit in any one of the electrical lines, the coolant temperature sensor will not get reference voltage. Meanwhile, the engine control unit detects the open circuit as excessive resistance and shows a coolant temperature as negative (-40).
Similarly, if there is a short circuit to the ground or to each other, the resistance in the circuit can drop. The engine control unit will measure the resistance and display a high coolant temperature even though the actual temperature is low.
The resistance can only be checked parallelly or with the help of a socket box. The electrical lines are repaired after identifying the area of open or short circuits. No parts are replaced as the fault P0115 is stored because of the electrical lines.
- Coolant temperature sensor
The coolant temperature sensor is NTC type, it is located very near to the engine block. The electrical pins can get corrosive if there is a small leak from the temperature sensor O rings. In a few cases, even the NTC element can develop excessive internal resistance.
Replacing the affected sensor is the only remedy for a faulty coolant temperature sensor. The O-ring that seals the temperature sensor and the cylinder head is also replaced.
- Engine control unit
The engine control unit provided the reference voltage for the operation of the coolant temperature sensor. When the reference voltage is not okay or it is under-voltage, the measurement of the coolant temperature is also implausible.
Therefore, if the fault P0115 is present even after replacing the coolant temperature sensor and the electrical lines are okay. Then the fault is present due to a faulty engine control unit and it must be replaced.
How to diagnose a P0115 fault code?
The following steps are performed in order to diagnose a P0115 fault code:
- Perform a visual inspection of the coolant temperature sensor and the electrical lines.
- If the visual inspection is okay, then proceed to measure the internal resistance of the coolant temperature sensor.
- The engine temperature should be less than 80 degrees as the sensor can be located very close to the cylinder head.
- If the internal resistance of the coolant temperature sensor is less than 10 M ohms then proceed with the next step.
- Remove the electrical coupler of the coolant temperature sensor and connect a decade resistor box to it.
- Turn on the ignition, do not start the engine. Apply resistance from the decade resistor box and observe the coolant temperature in the instrument cluster.
- If the coolant temperature changes in the instrument cluster as per the specified table provided by the manufacturer, it means that the electrical lines and the engine control unit are functioning correctly.
While diagnosing the P0115 fault code, it is recommended to wear tight-fitted clothing as the radiator fan can turn on automatically. The fan can also turn at full speed when the decade resistor box is connected to the electrical coupler of the coolant temperature sensor. Hence, remove or disconnect the electrical coupler of the cooling fan.
Can you drive with a DTC P0115?
Yes, the vehicle can be driven with the P0115 fault code present in the DTC memory of the engine control unit. However, the control unit will actuate the check engine light and reduce the performance of the engine to protect the engine from overheating.
Driving a car with a malfunctioning coolant temperature sensor is not good, the coolant can overheat and the driver will remain unaware as the coolant temperature in the cluster is incorrect.
There can be damage to the cylinder head and cylinder head gasket if the engine is overheated. As the coolant dries up in the coolant system, moving components like pistons, and connecting rods can also seize up.
Therefore, rectifying the fault code P0115 as soon as it occurs can prevent some very expensive repair bills. The average cost of the coolant temperature sensor is 150 to 200 USD and most dealerships will charge 50-100 USD to diagnose and rectify the P 0115 fault code.
Other articles about Vehicle Fault Codes you may be interested in
What are the symptoms of the P0715 fault code?
What are the symptoms of the P2422 fault code?
Why does the fault code P0400 occur in diesel cars?
Conclusion
This blog explained the causes and remedies for the P0115 fault code. Fixing the P0115 fault code is relatively easy and not expensive. If the fault is ignored then there can be catastrophic damage to the engine. In some cases, the cylinder head can also get warped due to overheating. Hence, the P0115 fault code should be fixed as soon as possible.
