P0118 Engine Coolant Temperature Sensor 1 Circuit High
The engine coolant temperature sensor 1 is an NTC thermistor: the control module feeds 5 V through a resistor, and as the coolant warms the resistance and signal voltage fall. Its value drives cold-start enrichment, idle speed, ignition timing, the radiator fan and the release of closed-loop mixture control. Some Ford engines measure cylinder head temperature instead and calculate a coolant value from it. P0118 means the signal voltage sits near the 5 V reference, so the sensor would report about −40 °C. Typical causes are an unplugged or corroded connector, a broken signal or sensor ground wire, an open sensor or a short to voltage. Limits: GM above 134 kilohms (only after 10 s running, so genuine cold is not misread), Ford petrol above 4.96 V for 5 s, Ford diesel above 4.91 V, GM/Saab below −39 °C for 4 s. Without a substitute strategy the mixture would be far too rich; GM/Saab run the fan and switch off the A/C. On Ford engines with a head temperature sensor the equivalent is P1289.
Likely causes
Symptoms
- Check engine light on
- Radiator fan runs constantly
- Temperature gauge stays cold
- Rich mixture, higher consumption, possibly black smoke or fuel smell
- Raised idle speed
Repair estimate
Typical range across vehicles; the real cost depends on what the testing finds.
What to check first
- Check connector
- Check for an open circuit
- Replace sensor if out of range
Parts involved
Related codes
References: law.cornell.edu, gsi.ext.gm.com, fordservicecontent.com, saabwisonline.com.