Engine Coolant Temperature Sensor 1 Circuit High
What it means
P0118 means the engine coolant temperature sensor circuit is reading high voltage, which the ECM interprets as an implausibly cold engine — the opposite of overheating. It's typically an open or high-resistance in the sensor or its wiring. Because the ECM thinks the engine never warms up, it over-fuels the mixture, hurting economy, causing hard warm starts, and possibly keeping the cooling fans off when they're needed.
Can I keep driving? Drive with care — the ECM thinks the engine is stone cold, so it over-fuels; expect poor economy and hard warm starts, and cooling fans may not run when they should.
Common symptoms
What you'll notice
- Check-engine light on
- Coolant-temp gauge reads very low or dead-cold on a scan tool (e.g. -40°)
- Hard starting when warm and a rich-running, poor fuel economy
- Black smoke or rough running from the over-rich mixture
Common causes
Most likely first
- Open or high-resistance in the ECT sensor wiring — most common for a 'high' signalLikely
- Failed ECT sensor reading openCommon
- Corroded or disconnected ECT sensor connectorCommon
- Rarely, an ECM faultLess often
How to diagnose & fix
Work through it in order
- Scan the live coolant-temp value; a reading stuck at the extreme cold end confirms an open/high circuit.
- Inspect the ECT connector and wiring for a break, corrosion, or a loose pin.
- Briefly jumper the sensor's two terminals together — if the reading swings to hot, the sensor is open and should be replaced.
- Replace the ECT sensor if it tests open, then clear and re-test.
FAQ
Quick answers
Is it safe to drive with P0118?+
Can I just clear P0118?+
Related codes
Often seen with P0118
Read & clear P0118 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.