What causes fault code P0118?

• Most likely: Faulty Engine Coolant Temperature (ECT) sensor (internal open circuit) • Likely: Corrosion or poor connection in the sensor connector • Possible: Wiring harness break in the signal or ground wire between the ECM and sensor • Rare: Faulty Engine Control Module (ECM) – internal failure of the reference voltage circuit

How do you fix fault code P0118?

STEP 1: Visually inspect the ECT sensor connector for corrosion or loose pins. Clean/repair as necessary. STEP 2: Use a scan tool to monitor live data. If the temperature reads -40°C, the circuit is likely open. Short the two pins in the connector with a jumper wire; if the reading jumps to +140°C, the wiring is intact and the sensor is faulty. STEP 3: Measure the reference voltage at the connector (should be approx. 5V) and check the ground circuit continuity to the chassis/engine block (should be < 0.1 Ohm). STEP 4: Measure the sensor resistance with a multimeter. At 68°F (20°C), resistance should be approx. 2000-3000 Ohms; at 176°F (80°C), approx. 200-300 Ohms. If reading is Infinite/OL, replace the ECT sensor.

Powertrain (Engine & Gearbox)Standard EOBD

P0118 - Engine Coolant Temperature Sensor 1 Circuit High

Q: What does fault code P0118 mean?

The P0118 code is triggered when the Engine Control Module (ECM) detects a voltage from the Engine Coolant Temperature (ECT) sensor that is higher than the expected range (typically exceeding 4.6V to 4.9V). The ECM supplies a 5V reference to the sensor, which is an NTC (Negative Temperature Coefficient) thermistor. As coolant temperature increases, the sensor's resistance decreases, pulling the voltage signal towards ground. A high voltage signal (near 5V) indicates an open circuit or extremely high resistance, leading the ECM to believe the coolant is colder than -40°C. Consequently, the ECM enters a failsafe mode, often substituting a fixed temperature value and running the cooling fans at high speed to prevent overheating.

Q: What are the symptoms of fault code P0118?

• Check Engine Light (MIL) illuminated • Cooling fans running at high speed constantly • Hard to start (especially when cold) • Rough idling or engine stumbling • Temperature gauge in instrument cluster reads inaccurate or stays at minimum • Failed emissions test

Last updated:1 February 2026

Related:ECT sensor (Engine Coolant Temperature), Engine Control Module (ECM), Wiring harness / Connectors, Thermostat

System

Powertrain (Engine & Gearbox)

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Technical Description

The P0118 code is triggered when the Engine Control Module (ECM) detects a voltage from the Engine Coolant Temperature (ECT) sensor that is higher than the expected range (typically exceeding 4.6V to 4.9V). The ECM supplies a 5V reference to the sensor, which is an NTC (Negative Temperature Coefficient) thermistor. As coolant temperature increases, the sensor's resistance decreases, pulling the voltage signal towards ground. A high voltage signal (near 5V) indicates an open circuit or extremely high resistance, leading the ECM to believe the coolant is colder than -40°C. Consequently, the ECM enters a failsafe mode, often substituting a fixed temperature value and running the cooling fans at high speed to prevent overheating.

Possible Causes

Faulty Engine Coolant Temperature (ECT) sensor (internal open circuit)

Corrosion or poor connection in the sensor connector

Wiring harness break in the signal or ground wire between the ECM and sensor

Faulty Engine Control Module (ECM) – internal failure of the reference voltage circuit

Symptoms

Check Engine Light (MIL) illuminated

Cooling fans running at high speed constantly

Hard to start (especially when cold)

Rough idling or engine stumbling

Temperature gauge in instrument cluster reads inaccurate or stays at minimum

Failed emissions test

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P0118 - Engine Coolant Temperature Sensor 1 Circuit High

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P0118 - Engine Coolant Temperature Sensor 1 Circuit High

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