P0118
Engine Coolant Temperature Sensor High Input
The ECT signal voltage is above threshold, which the ECU interprets as an implausibly cold temperature (open-circuit-like behaviour). When this code is active, the ECU runs a default cold-start enrichment that wastes fuel and may cause black smoke.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: easy
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
Common causes
Ordered by how often each turns out to be the cause.
- Open circuit in sensor or wiringCommon
- Corroded sensor connectorPossible
Parts involved
- Coolant Temperature Sensor
- Coolant Temperature Sensor Wiring
P0118 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.