P0115: Engine Coolant Temperature Sensor 1 Circuit
P0115 means Engine Coolant Temperature Sensor 1 Circuit. The engine coolant temperature (ECT) sensor tells the engine computer how warm the engine is. It affects fuel mixture, idle speed, cooling fan control and the temperature gauge. The control module has detected a fault in the circuit itself, such as an open circuit, a short, or a signal outside the electrical limits it expects. A low voltage reads as hot; a high voltage reads as cold.
Can I keep driving? Drivable with care. Diagnose soon: performance, economy or other components can be affected.
Common symptoms
- Check engine light on
- Cooling fans running constantly (common fail-safe)
- Hard starting or rough cold running
- Poor fuel economy or black smoke
- Temperature gauge reading wrong
Possible causes
- Failed coolant temperature sensor
- Damaged connector or wiring
- Low coolant level leaving the sensor in air
- Thermostat fault (range/performance codes)
How to diagnose P0115
- Check coolant level with the engine cold.
- Compare the ECT reading with the IAT reading after the car has stood overnight: they should be within a few degrees.
- Check the sensor connector, signal and ground.
- Measure sensor resistance at a known temperature against the specification.
Never open a hot cooling system. Wait until the engine has cooled down.
What the code means, digit by digit
Generic codes have the same definition on every OBD-II vehicle (petrol cars from model year 1996 in the US, 2001 in the EU; diesels later). Manufacturers can add their own enhanced codes; those are not listed here.
Symptoms linked to P0115
Related codes
General guidance only. Fault codes describe what the engine computer saw, not which part failed: confirm with testing before replacing parts, and follow the vehicle's service information.