P0118 - Engine Coolant Temperature Circuit High Input

P0118 means the coolant temperature signal is reading colder than expected because the circuit is open or the signal is stuck high. It is not automatically just a bad coolant temperature sensor; an unplugged sensor, damaged wiring, connector corrosion, open circuit, failed sensor, or less common PCM and reference-voltage issue can set the same code.

Treat the code as the starting point: compare symptoms, likely causes, and repair paths before pricing parts or moving into an estimate.

Code Overview

P0118 means the coolant temperature signal is reading colder than expected because the circuit is open or the signal is stuck high. It is not automatically just a bad coolant temperature sensor; an unplugged sensor, damaged wiring, connector corrosion, open circuit, failed sensor, or less common PCM and reference-voltage issue can set the same code.

Common Causes

• Unplugged coolant temperature sensor
• Open circuit or broken wiring in the ECT sensor path
• Failed engine coolant temperature sensor
• Coolant intrusion or corrosion at the connector

Symptoms

• Hard cold starts or rich-running behavior
• Cooling-fan operation may be abnormal
• Fuel economy can drop while warm-up strategy stays on too long

Diagnostic Steps

1. Make sure the coolant temperature sensor connector is fully seated
2. Inspect the wiring for breaks, corrosion, or coolant intrusion
3. Compare ECT reading to ambient temperature on a cold engine
4. Check sensor resistance and circuit integrity before replacing parts

Diagnostic Insight

P0118 should be treated as a coolant-temperature circuit-high fault before the sensor itself is condemned.

• A cold-looking ECT reading on a warm engine often points to an open circuit or poor connection.
• Connector corrosion, coolant intrusion, or broken wiring can mimic a failed sensor.
• Cold-engine scan data should be compared to ambient temperature before parts are replaced.

Repair Difficulty

Easy

General difficulty estimate for the most common repair path.

Likely Repairs & Cost Guides

Use symptoms, scan data, and quick checks to confirm the likely repair path before pricing parts. The estimator helps compare repair paths before replacing anything unnecessarily.

Related OBD Codes

Browse nearby code-family pages when the same fault pattern overlaps across systems, such as misfire, lean fuel trim, EVAP sealing, cooling, or charging faults.

Next Steps

Move from code lookup to diagnosis, then estimate the likely repair only after symptoms, causes, and checks point to the same path.