P0118 – Engine Coolant Temperature Circuit High Input

What does P0118 mean?

The engine coolant temperature sensor circuit is reading high voltage, which the computer interprets as an implausibly cold engine — often as though the coolant were far below ambient temperature. This usually indicates an open circuit rather than a genuine temperature.

What causes P0118?

Listed from most to least common:

1. 1Failed ECT sensor (open internally)
2. 2Disconnected sensor connector
3. 3Broken or corroded signal wire
4. 4Poor ground connection
5. 5Corrosion in the connector

Code breakdown

Common symptoms

• Check engine light on
• Rich running, black smoke, strong fuel smell
• Noticeably poor fuel economy
• Cooling fans may not turn on
• Temperature gauge reads low or does not move
• Hard starting when warm

How to diagnose

1. 1Check that the connector is fully seated — this code often follows recent cooling system work.
2. 2Inspect the wiring for breaks or corrosion.
3. 3Compare live ECT reading to ambient temperature with the engine fully cold; they should match closely.
4. 4Jumper the signal and ground terminals at the connector. If the reading swings to maximum temperature, the wiring is good and the sensor has failed.
5. 5Measure sensor resistance against the specification table.

The computer will over-enrich the mixture believing the engine is cold, wasting fuel and potentially fouling plugs. Cooling fans may also fail to engage when needed, risking overheating.

Common fixes and estimated costs

Costs are rough US estimates including parts and labor. Actual pricing varies significantly by vehicle, region, and shop rates.

Good to know

Because the fans may not engage with this code active, keep an eye on actual engine temperature until it is repaired. The sensor itself is usually inexpensive and accessible.

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