Faulty engine coolant temperature sensor giving an erratic or out-of-range signal; Corroded, loose or damaged sensor connector or terminals; Damaged wiring to the sensor, such as chafing, open circuit, or short to earth or voltage; Low coolant level or air in the cooling system leaving the sensor reading incorrectly; Thermostat stuck open, so the engine never reaches a plausible operating temperature; Fault in the engine control unit's sensor reference voltage or input circuit.

P0115

Engine Coolant Temperature Sensor 1 Circuit.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
1: AreaFuel and air metering
15: FaultThe specific fault in that area

What it means

P0115 is the generic code for a fault in the engine coolant temperature (ECT) sensor 1 circuit. The engine control unit has seen a signal that is out of range, implausible or erratic compared with expected values, such as a reading that does not match engine running time, load or other temperature inputs. The code covers the sensor, its wiring and connector, and the reference and signal circuits, so the exact trigger conditions depend on the vehicle.

Symptoms

Engine warning light on
Hard starting, especially when cold or when hot
Cooling fan running constantly or not coming on when it should
Poor fuel economy or rich running with black smoke
Rough idle or hesitation
Temperature gauge reading erratically, stuck or not moving

Possible causes

Faulty engine coolant temperature sensor giving an erratic or out-of-range signallikely
Corroded, loose or damaged sensor connector or terminalslikely
Damaged wiring to the sensor, such as chafing, open circuit, or short to earth or voltagepossible
Low coolant level or air in the cooling system leaving the sensor reading incorrectlypossible
Thermostat stuck open, so the engine never reaches a plausible operating temperaturepossible
Fault in the engine control unit's sensor reference voltage or input circuitrare

How to diagnose

Read the stored codes and freeze frame data, and note any other temperature or sensor codes that are present
Check the coolant level and condition when the engine is cold, and look for leaks
Inspect the sensor connector and wiring for corrosion, damage, bent pins and chafing, and wiggle the harness while watching live data
Compare the live coolant temperature reading on a cold engine with the intake air temperature; they should be close after the car has sat overnight
Watch the live coolant temperature as the engine warms up and check that it rises smoothly to a sensible operating temperature without jumps or dropouts
Measure the sensor resistance against the manufacturer's temperature table, and check the reference voltage and signal circuit at the connector
Check the thermostat is opening correctly if the engine is slow to warm up or does not reach normal temperature
Test the wiring for continuity and for shorts to earth or voltage between the sensor and the control unit

How to fix

Replace the coolant temperature sensor if its resistance or signal is out of specification
Clean or repair the connector and terminals, or replace the connector if it is damaged
Repair or replace damaged wiring in the sensor circuit
Top up and correctly bleed the cooling system, and repair any leak found
Replace a faulty thermostat
Repair or replace the engine control unit only after the sensor, wiring and supply have been proven good

Can I keep driving?

Medium severity

The car can usually be driven short distances, but fix it soon because the control unit may run the engine rich, use the fan wrongly or lose accurate overheating protection. If the temperature gauge reads high or the engine is overheating, stop driving and have it checked.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.