Faulty engine coolant temperature sensor (internal open circuit); Corroded, loose or damaged sensor connector; Open circuit or high resistance in the signal or ground wire between sensor and ECU; Signal wire shorted to the reference voltage supply; Sensor unplugged after recent work or damaged by previous repair; Fault inside the ECU, such as a damaged internal sensor circuit.

P0118

Engine Coolant Temperature Sensor 1 Circuit High.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
1: AreaFuel and air metering
18: FaultThe specific fault in that area

What it means

P0118 means the engine control unit has detected the signal voltage from engine coolant temperature sensor 1 is higher than expected, which equates to an implausibly low temperature reading. This normally points to an open circuit, a poor connection or a failed sensor. The ECU sees the high voltage and usually substitutes a default coolant temperature value.

Symptoms

Engine warning light on
Hard starting, especially when the engine is warm
Rich running, black smoke or poor fuel economy
Cooling fans running constantly or not at the right time
Temperature gauge reading cold or not moving
Rough idle or poor performance

Possible causes

Faulty engine coolant temperature sensor (internal open circuit)likely
Corroded, loose or damaged sensor connectorlikely
Open circuit or high resistance in the signal or ground wire between sensor and ECUlikely
Signal wire shorted to the reference voltage supplypossible
Sensor unplugged after recent work or damaged by previous repairpossible
Fault inside the ECU, such as a damaged internal sensor circuitrare

How to diagnose

Read the stored codes and freeze frame data to see engine conditions when the code set and check for any other related codes.
Compare the live coolant temperature reading with the true engine temperature on a cold engine; a reading at the minimum value suggests an open circuit.
Inspect the sensor, connector and wiring for damage, corrosion, bent pins, water ingress and chafing.
With the key on, unplug the sensor and check the reading; then bridge the two sensor terminals of the harness connector and see whether the reading swings to maximum temperature, which points to the sensor or wiring.
Check the reference voltage and ground at the harness connector against the manufacturer's specification for your model.
Measure the sensor resistance at a known temperature and compare it with the manufacturer's data.
Test the wiring between the sensor and ECU for continuity, high resistance and shorts to voltage, referring to the model-specific wiring diagram.
If the sensor and wiring test good, consider checking the ECU connector and ECU, following the manufacturer's procedure.

How to fix

Replace the coolant temperature sensor if its resistance is out of specification or it is open circuit.
Clean or repair the connector terminals, or replace the connector if it is corroded or damaged.
Repair or replace any open, high-resistance or shorted wiring.
Secure the connector fully and re-route or protect any harness that has been chafing.
Repair or replace the ECU only after all other causes are ruled out.

Can I keep driving?

Medium severity

The car can usually be driven for a short time, but it should be fixed soon. The ECU is using a default temperature, so running may be rich, fan control may be affected and the car may be hard to start, and the temperature gauge may not be reliable to warn of overheating.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.