P0118 means the engine control module is seeing a higher-than-expected voltage or resistance from Engine Coolant Temperature (ECT) sensor circuit 1, which it interprets as an abnormally low coolant temperature or an open/high-resistance condition. The ECT sensor is a thermistor whose resistance drops as coolant warms up, so a 'high' signal usually points to an open signal path or a sensor that is not reading correctly.
The thermistor inside the sensor can crack, corrode, or internally open, producing a high-resistance signal that the PCM reads as circuit high. This is the single most common cause of P0118.
An open signal wire, poor pin connection, corrosion, or a short-to-voltage on the signal or 5 V reference circuit can push the reading above the normal range.
If the sensor tip is exposed to air instead of coolant, it can read much colder than the actual engine, producing a voltage that sets P0118.
Using the wrong ECT sensor for the application, or an aftermarket adapter or thermostat that changes coolant flow, can produce an out-of-range signal.
A rare internal PCM fault or corrupted calibration can misread a normal sensor signal. This should be considered only after the sensor, wiring, and cooling system have been verified.
It is usually safe for short trips, but the engine may run rich and fuel economy can suffer. Monitor the temperature gauge closely to avoid real overheating, since the sensor may not report accurately.
No. P0117 is 'Circuit Low,' meaning the signal voltage is too low or resistance is too low, while P0118 is 'Circuit High,' meaning the signal voltage is too high or resistance is too high.
Usually not. The vast majority of P0118 codes are caused by a faulty ECT sensor, wiring/connector problems, or low coolant. PCM replacement is uncommon and should only be considered after the sensor and circuit are fully tested.
Yes. If the sensor tip is not fully submerged in coolant, it can read air temperature or a colder value, producing a high voltage that sets the code.