P0118: Engine Coolant Temperature Sensor 1 Circuit High

What does P0118 mean?

The coolant temperature (ECT) sensor helps the ECM decide how much fuel to add, when to change idle speed and, on many cars, when to run the cooling fans.

A circuit high code means the signal voltage is above the normal range. On a typical sensor that looks like an extremely cold engine, which is what an open circuit, an unplugged connector or a failed sensor produces. Because the ECM thinks the engine is cold, it can add extra fuel.

Common symptoms

• Check engine light on
• Hard starting
• Poorer fuel economy
• Black smoke from the exhaust in some cases
• Sometimes no other noticeable symptom

Common causes

The causes below are the ones commonly seen with P0118. The actual cause can vary by vehicle, engine and mileage, so treat this list as a starting point, not a diagnosis.

• A loose, corroded or unplugged sensor connector
• An open circuit in the signal or ground wire
• A faulty ECT sensor

What to check with a scanner

• In Live data, look at the coolant temperature on a warm engine. A reading stuck at an implausibly cold value suggests an open circuit.
• On a cold engine, compare coolant temperature with intake air temperature. They should be similar.
• Inspect the connector and wiring before replacing the sensor.
• Clear the code after the repair and watch the reading rise as the engine warms.

How to read and clear P0118 with OBD Master Pro

1. Plug in your adapter. Connect a generic ELM327 (or ELM329) OBD2 adapter to the diagnostic port, usually under the dashboard on the driver's side. Bluetooth Classic, Bluetooth LE, Wi-Fi and USB OTG adapters are supported.
2. Switch the ignition on. Turn the key to ON or start the engine, depending on your car.
3. Connect in OBD Master Pro. Open the app, go to Connection and pick your adapter. Classic Bluetooth adapters connect from the Bluetooth tab, Bluetooth LE adapters from the BLE tab with no pairing needed. Use the generic OBD2 / EOBD profile or your car's brand profile.
4. Read the codes. Open the fault codes screen to see stored, pending and permanent codes. Confirm P0118 is listed and note any other codes, since they often explain the cause.
5. Check the details. Look at freeze frame data, Live data and, where your car supports it, Mode 06 test results (see the checks above).
6. Fix, then clear. After the repair, clear the codes to turn off the check engine light. Clearing also resets the emissions readiness monitors, so drive normally and check the readiness screen before any inspection. Permanent codes cannot be erased with a scan tool; they clear on their own once the car's self test passes.

Tip: you can also look up P0118 and other codes in the app's DTC search without connecting to a car. If your adapter will not connect, use Connection > Share connection log and email it to support. Many cheap ELM327 clones only handle basic engine OBD2, which is all you need for P0118.

When to see a mechanic

Get professional help if any of these apply:

• The wiring and connector look fine but the code returns
• The engine runs very rich or stalls
• You need voltage and ground tests at the sensor

If you are unsure, a qualified technician can confirm the cause with tests such as smoke testing, pressure testing or scope readings. Clearing a code does not repair the fault.

Related trouble codes

See all OBD2 trouble code guides

Related guides

Check engine light on: what to do · How to read an OBD2 code (P, B, C, U) · Stored, pending and permanent codes and freeze frame · OBD2 readiness monitors explained · OBD2 glossary · OBD2 by car brand