What it means
The module detects a high input from the engine coolant temperature sensor circuit. With a typical NTC sensor, this may correspond to an implausibly low temperature.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
The engine coolant temperature sensor reports coolant temperature so the ECU can manage fueling, warm-up, and cooling-fan operation. P0118 sets when the sensor circuit input is too high; with a typical negative-temperature-coefficient sensor, an open circuit can be interpreted as extremely cold coolant.
Common causes
Most likely first.
- Open circuit in sensor or wiring
- Corroded sensor connector
Symptoms
- Check engine light on
- Cooling fan runs constantly or not at all
- Cabin heater weak or excessively hot
- Possible long warm-up time or hard cold start
How to diagnose it
- Record freeze-frame data and compare live coolant temperature with ambient temperature on a fully cold engine to identify an implausibly low reading.
- Inspect the coolant temperature sensor connector and wiring for corrosion, loose pins, breaks, or contact with hot or moving parts.
- Check the live temperature as the engine warms and compare it with other temperature information, noting whether it stays fixed or changes implausibly.
- Test the sensor circuit for an open, poor connection, or faulty sensor using the wiring information and manufacturer's specification.
- Verify actual engine temperature and cooling-fan operation independently so a circuit fault does not hide a genuine overheating problem.
Common fixes
- Reconnect or clean the coolant temperature sensor connector.
- Repair an open or damaged sensor wire.
- Replace the coolant temperature sensor if it fails the specified test.
- Repair a confirmed fan-control fault if the fans do not operate as required.
Can I keep driving?
Check the temperature gauge and warning lights before driving; stop if the engine overheats or coolant is lost. A false cold reading can disrupt fueling and fan control, so avoid long trips until the circuit and actual cooling operation are confirmed.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.