P0115: Engine Coolant Temperature Sensor 1 Circuit
The engine computer is getting an erratic or impossible signal from the coolant temperature sensor.
What P0115 means
The engine coolant temperature sensor is a thermistor screwed into a coolant passage. Its voltage is high when the engine is cold and drops as it warms up. The engine computer uses that reading for fuel mixture, spark timing, idle speed and often the radiator fans. RepairPal explains that P0115 sets when the computer sees an erratic or irrational signal from this circuit. Common causes are a failed sensor, corroded or loose wiring and old rusty coolant. RepairPal warns that sensors are sometimes replaced when the actual problem was rusty coolant, a poor connection or a real cooling system fault. Many drivers notice nothing at first, but the computer may run the engine rich or keep the fans on to be safe. Shops compare the ECT reading to the intake air temperature on a cold engine, test the sensor resistance against a temperature chart, and wiggle the harness while watching live data. Keep an eye on your temperature gauge until it is fixed.
Common symptoms
- Check engine light on
- Hard starting, especially when cold or hot
- Poor fuel economy
- Cooling fans running all the time or not at all
- Temperature gauge reading wrong or erratic
Most common causes
- Faulty engine coolant temperature (ECT) sensor
- Corroded or loose sensor connector
- Damaged wiring to the sensor
- Old, rusty coolant corroding the sensor tip
- Low coolant level leaving the sensor in air
What to check yourself first
With the engine cold, check the coolant level and color; brown or rusty coolant is a warning sign. Unplug the ECT sensor and look for green corrosion on the pins. On a cold engine that sat overnight, a scan tool should show coolant temperature close to the outside air temperature.
Typical fixes and what they cost
| Fix | Typical cost | Note |
|---|---|---|
| Replace engine coolant temperature sensor | $499-$585 | RepairPal average for the sensor, which is often easy to reach on the thermostat housing or cylinder head. |
| Electrical diagnosis to find a wiring or connector fault | $122-$179 | For corroded pins or a chafed wire, which RepairPal says often get missed. |
| Coolant drain and refill | $440-$512 | Rusty coolant can foul the sensor, so a flush may be part of the fix. |
Typical US price ranges for parts and labor, not quotes. Your shop should confirm the cause with a diagnostic before replacing parts.
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Related codes
- P0116 Engine Coolant Temperature Sensor 1 Circuit Range/Performance
- P0117 Engine Coolant Temperature Sensor 1 Circuit Low
- P0118 Engine Coolant Temperature Sensor 1 Circuit High
- P0128 Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
- P0125 Insufficient Coolant Temperature for Closed Loop Fuel Control