P0115: Engine Coolant Temperature Sensor 1 Circuit
The engine control unit has detected a fault in the coolant temperature sensor circuit.
Normal risk
Usually drivable, but watch the temperature gauge. Have it checked soon.
Short answer · Can I keep driving with P0115?
Usually yes. The cooling fan may run constantly and starting may be harder. Watch the temperature gauge and have it checked soon.
What does P0115 mean?
Code P0115 is a general circuit fault for the engine coolant temperature sensor: the signal is missing or not valid. The control unit then assumes a substitute temperature and often runs the cooling fan as a precaution.
What the car does when the code is set
The check engine light comes on and the engine uses a substitute temperature. On many cars the cooling fan runs all the time, the temperature gauge stops working or shows a wrong value, and the engine may run a richer mixture, start less easily or idle faster than normal.
How the system works
The coolant temperature sensor is a thermistor screwed into the cooling system near the thermostat. The engine control unit sends it about 5 volts and measures the voltage left: high when the engine is cold, low when it is hot. This one value controls many things: the cold-start mixture, idle speed, ignition timing, the cooling fan, the temperature gauge on many cars and when the emission tests are allowed to run. A reading stuck at either end of the range is treated as a circuit fault, and a fixed substitute value is used instead.
Common symptoms
Possible causes
Listed without ranking. The cause on your car depends on make, engine and mileage — see the maker notes below.
- Faulty coolant temperature sensorThe sensor resistance fails, giving an impossible reading.
- Wiring short or open circuitA shorted wire gives a very hot reading; an open one a very cold reading.
- Corroded connectorCoolant leaks often corrode the connector.
- Engine control unit faultRare: the input in the control unit fails.
How it is diagnosed
- Compare the reading with realityOn a cold engine the coolant temperature should match the outside temperature. A fixed value of about −40 °C or more than 130 °C points to a circuit fault.
- Inspect the connector and wiringLook for corrosion, coolant in the connector and chafed wires.
- Measure the sensorCompare resistance at a known temperature and the reference voltage with the service data.
- Check the cooling fanMany engines run the fan permanently while this code is active. After the repair, confirm the fan switches normally.Do not open the coolant cap on a hot engine.
Questions about P0115
Why does the fan run all the time?
Without a valid coolant temperature, many cars switch the fan on permanently to prevent overheating.
Why is my cooling fan running all the time?
When the control unit cannot trust the coolant temperature, many engines switch the fan on permanently to protect the engine. It stops after the repair.
Can I replace the sensor myself?
On many engines it is easy to reach, but some coolant will be lost. Top up and bleed the cooling system as described in the service data.