P0118

Engine Coolant Temperature Sensor 1 Circuit High

Fuel & air metering Generic OBD-II 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.

High

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.

Real-world scan data

Seen in

6,609

scans in the past year

Found in

1.0%

of scans

1.9%

of scans that found faults

Appears alongside · share of P0118 scans

P0113in 20.1% of scans

P0115in 9.7% of scans

P0645in 8.1% of scans

Figures reflect only diagnostic scans recorded through our own apps and tools. Cars are typically scanned because a symptom or warning light prompted it, so these counts describe our scan records — not the incidence of faults in any model, and not the quality, reliability, or safety of any vehicle. No comparison between manufacturers or models is implied. All trademarks belong to their respective owners; no affiliation or endorsement is implied.

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?+

Short trips, cautiously. Because the ECM reads the engine as ice-cold it runs rich, which wastes fuel and can foul plugs; the cooling fans may also stay off, risking overheating in traffic. Fix it promptly.

Can I just clear P0118?+

It will return immediately if the circuit is still open. The reading needs to come back into a normal range — repair the wiring or replace the sensor first.

Read & clear P0118 yourself

Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.

▶ Get it on Google Play

✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX