P0118Engine coolant temperature sensor circuit high input
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— The engine will run rich (excess fuel vs air) and the fans may never engage, which can also mask a real overheat — repair soon to protect the catalyst and fuel economy.
General guidance only — not a substitute for a professional inspection or official roadside / out-of-service criteria (e.g. CVSA OOSC) in your jurisdiction.
Part of system: Engine cooling & overheat protection
Is the mirror image of a shorted coolant-temperature circuit: signal voltage sat at or near reference level, so the ECM concluded the engine was impossibly cold and fell back to a default temperature value. Open circuits dominate here — corroded terminals, a broken sensor ground, or a thermistor that has gone open internally. Jumper the connector's signal and ground pins with the key on and watch the scan-tool reading: if it snaps to maximum temperature, the wiring and computer are fine and the sensor is at fault, and if nothing changes the open lies in the harness or the ground path. Symptoms follow the false cold reading: long crank times, a rich warm-up strategy that never leans out, no cooling-fan command, and a heater that underperforms. Fuel economy usually drops noticeably.
Parts that may be involved
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Coolant temperature sensor and seal
Search for this part on Amazon(affiliate link)Terminal repair kit for the sensor connector
Search for this part on Amazon(affiliate link)Wiring repair supplies for the ground splice
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Vehicles often affected
Two-wire sensors on aluminium housings are prone to terminal corrosion, and shared sensor grounds mean one bad splice can set several temperature codes at once.
Typical symptoms
- Temperature gauge reading cold no matter how long it runs
- Rich running (excess fuel vs air, ~14.7:1 target) with black smoke and a fuel smell
- Cooling fans never commanded on
- Heater output cooler than expected in winter
- Noticeably worse fuel economy since the light appeared
Likely causes
- Signal wire open or a terminal backed out of the connector
- Sensor ground circuit open at the splice or the ECM
- Thermistor failed open inside the sensor body
- Corroded terminals raising resistance beyond the sensor range
- Rodent damage or an old repair splice that came apart
Recommended checks
- 1
Jumper signal to ground at the connector and watch the reading
- 2
Measure sensor resistance against the temperature chart
- 3
Check the shared sensor ground with a voltage drop test
- 4
Inspect the connector for pushed-out or spread terminals
- 5
Repair the open circuit and verify warm-up on a road test
DIY vs professional
DIY: jumper test, sensor swap, connector repair. Pro: ground circuit tracing through the harness and ECM-side testing.
See also
- P0117Check soonEngine coolant temperature sensor circuit low input
- P0128OK to driveCoolant thermostat below regulating temperature
- P0217Stop the vehicleEngine overtemperature conditionDiesel & RV
- P0113OK to driveIAT sensor circuit — high input
- P0101Check soonMass air flow sensor circuit range or performance problem
Related terms
Content reviewed by a heavy-duty technician. About our editorial approach