P0118: Engine Coolant Temperature Sensor Circuit High
P0118 means the engine computer is seeing a signal from the coolant temperature sensor that is stuck near 5 volts, which it reads as an extremely cold engine of about −40 °C. It is usually a cheap fix such as a loose or corroded connector or a broken wire, but it should be repaired soon because the engine may run too rich and the fan may run constantly.
You can drive, but get it checked soon
The car may still run, but the module may believe the engine is always cold and over-fuel it, which wastes fuel, can foul the exhaust system and may put the car into limp mode. The fan may also run nonstop. Keep trips short and have it fixed soon.
What it means
The coolant temperature sensor is a thermistor, a resistor whose resistance drops as the coolant warms. The engine control module sends 5 volts through a resistor and reads the voltage that results. A high reading means high resistance, which the module interprets as freezing cold. The module uses this value for extra fuel at cold start, idle speed, ignition timing, the radiator fan and the switch to closed-loop mixture control (where it adjusts fuel using exhaust sensor feedback). P0118 sets when the signal voltage stays near the 5 V reference, which points to an open circuit (a break in the sensor or its wiring) or a short to voltage. On some Ford engines the sensor measures cylinder head temperature and the module calculates coolant temperature from it; the equivalent code there is P1289. The code turns on the check engine light and can need a drive cycle (a set pattern of driving) to confirm a repair.
Likely causes
- Connector unplugged or corroded, contact openVery likely
A loose or corroded terminal breaks the circuit, so the signal voltage rises to the 5 V reference.
- Broken signal or sensor ground wirePossible
An open wire stops the current path, and the module sees the full 5 V as if the sensor were extremely cold.
- Sensor open internallyPossible
A sensor that has failed open has very high resistance, which gives the same high voltage reading.
- Signal wire shorted to 5 V or battery voltageLess likely
A short to voltage pushes the signal up to or above the reference level.
- Engine control module faultyLess likely
A fault inside the module can misread or mis-supply the signal, though this is uncommon compared with wiring and sensor faults.
What you may notice
- Check engine light on
- Radiator fan runs constantly
- Temperature gauge stays cold
- Rich mixture, higher fuel consumption, possibly black smoke or a fuel smell
- Raised idle speed
What to check, cheapest first
- 1Look at the coolant temperature sensor connector. Make sure it is fully plugged in, and check for corrosion, bent pins or a damaged locking clip. · you can do it
- 2Inspect the wiring from the sensor along its path for breaks, chafing, or damage near hot or moving parts. · you can do it
- 3Unplug and reseat the connector, clean any corrosion, then clear the code and see whether it returns after a drive cycle. · you can do it
- 4Read live data with a scan tool and compare the coolant temperature value with the real engine temperature. A reading of about −40 °C on a warm engine confirms the fault. · mechanic
- 5Test the sensor resistance and the 5 V reference and ground at the connector to separate a failed sensor from a wiring fault. · mechanic
- 6Check the signal wire for a short to voltage and for continuity back to the control module. Test the module only after the sensor and wiring are cleared. · mechanic
Related codes
Frequently asked questions
Can I drive with a P0118 code?+
Short trips are usually possible, but the engine may run too rich, use more fuel and put the car into limp mode. Get it repaired soon.
Can a loose gas cap cause P0118?+
No. A gas cap affects evaporative emissions codes. P0118 points to the coolant temperature sensor circuit, most often its connector or wiring.
Will P0118 pass an emissions test?+
Unlikely. The check engine light is on and the code is emissions relevant, so the car will normally fail until it is repaired and the system has completed its drive cycle.
Is P0118 hard to fix?+
Often not. It is rated an easy repair that many owners can do themselves, typically taking about 0.3 to 1.5 hours.
Written with the help of AI from the public OBDex definition (CC0). Each make may use the code differently. Updated: 2026-11-10