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.
High severity. Can I keep driving?Do not trust the gauge: many cars turn on the fan as protection, but not all do, and the engine can overheat without warning. If you have no other option, make the shortest possible trip while watching for steam or a loss of cabin heat; at the slightest doubt, stop and call a tow truck.
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
Most likely causes
- Connector unplugged or corroded, contact open
- Broken signal or sensor ground wire
- Sensor open internally
- Signal wire shorted to 5 V or battery voltage
- Engine control module faulty
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: easy
- It can trigger limp mode.
- It affects emissions.
What each character of P0118 means
- P: Powertrain: engine and transmission
- 0: Generic code: same on every make
- 1: Air and fuel
- 18: Specific fault in that system
Frequently asked questions
Can I keep driving with code P0118?
Do not trust the gauge: many cars turn on the fan as protection, but not all do, and the engine can overheat without warning. If you have no other option, make the shortest possible trip while watching for steam or a loss of cabin heat; at the slightest doubt, stop and call a tow truck.
What causes code P0118?
The most common causes are: connector unplugged or corroded, contact open; broken signal or sensor ground wire; sensor open internally.
Related codes
- P0117Engine Coolant Temperature Sensor 1 Circuit Low
- P0115Engine Coolant Temperature Sensor 1 Circuit
- P0116Engine Coolant Temperature Sensor 1 Circuit Range/Performance
- P0119Engine Coolant Temperature Sensor 1 Circuit Intermittent/Erratic
- P2185Engine Coolant Temperature Sensor 2 Circuit High
Sources
- law.cornell.edu: 13 CCR 1968.2
- gsi.ext.gm.com: 18OBDG04 Engine Diagnostics
- fordservicecontent.com: OBD Operations Summary Gasoline 2020 2023
- saabwisonline.com: dtc p0117 p0118 or p0119
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.