P0118 Engine Coolant Temperature Sensor Circuit High Input

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P0118 Engine Coolant Temperature Sensor Circuit High Input

What Does Code P0118 Mean?

Diagnostic Trouble Code P0118 indicates that the Engine Control Module (ECM/PCM) has detected an abnormally high voltage signal from the Engine Coolant Temperature (ECT) sensor circuit. Since the ECT sensor is a negative temperature coefficient (NTC) thermistor, a high voltage signal is interpreted as an extremely low coolant temperature. This is commonly caused by an open circuit, disconnected sensor, or faulty wiring.

Symptoms

  • Check Engine Light (MIL) illuminated
  • Hard starting
  • Poor fuel economy
  • Rich fuel mixture
  • Rough idle
  • Cooling fan may not operate correctly
  • Incorrect coolant temperature reading on the scan tool

Possible Causes

  • Faulty Engine Coolant Temperature (ECT) sensor
  • Disconnected or loose ECT connector
  • Open circuit in the signal wire
  • Broken wiring harness
  • Poor connector terminal contact
  • ECM internal fault (rare)

Typical Circuit

ECM (5V Reference)
        │
        ├────────────► ECT Sensor
        │
Ground ─┘
        │
Signal ─────────────► ECM

Signal Flow

Coolant Temperature
        │
        ▼
ECT Sensor
        │
Voltage Signal
        ▼
ECM
        │
Fuel Injection
Ignition Timing
Cooling Fan Control

How to Diagnose

1. Visual Inspection

  • Check that the ECT sensor connector is fully seated.
  • Inspect the wiring harness for broken or disconnected wires.
  • Look for corrosion inside the connector terminals.

2. Live Data Test (Autel)

  • Open Diagnosis → Engine → Live Data.
  • Select Engine Coolant Temperature (ECT).
  • If the scan tool displays approximately -40°C (-40°F), the ECM is receiving a high-voltage signal caused by an open circuit.
  • Reconnect the sensor or repair the wiring if the reading does not change after inspection.
  • Compare the ECT value with the Intake Air Temperature (IAT) on a cold engine. Both readings should be nearly identical.

3. Multimeter Test

  • Verify the 5V reference voltage.
  • Check sensor ground continuity.
  • Measure sensor resistance according to the manufacturer's temperature chart.
  • Check for an open circuit between the ECM and the ECT sensor.

4. Smoke Test (If Applicable)

A smoke test is generally not required for P0118 because it is an electrical circuit fault. Perform a smoke test only if additional intake-related symptoms indicate a possible vacuum leak.

Common Repairs

  • Replace the faulty ECT sensor.
  • Repair open or broken wiring.
  • Reconnect or replace the damaged connector.
  • Repair poor terminal contact.
  • Clear the DTC and verify normal coolant temperature readings during a road test.

Severity

Moderate. Incorrect coolant temperature information can cause excessive fuel consumption, poor engine performance, increased emissions, and improper cooling fan operation.

Important Note

The ECT sensor wiring, resistance values, connector pinout, and voltage specifications vary by vehicle manufacturer. Always consult the factory service information for the specific vehicle before performing diagnostics or repairs.

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