Engine Coolant Temperature Circuit High Input
The P0118 code indicates that the Engine Control Module (ECM) has detected a voltage signal from the Engine Coolant Temperature (ECT) sensor that is higher than the expected normal operating range. This typically means the sensor is reading an extremely cold temperature, usually -40 degrees, pointing to an open circuit or a faulty sensor.
While the vehicle may still run, the engine computer will substitute a default temperature value, leading to poor fuel economy and rough idling. It should be diagnosed soon to prevent drivability issues and ensure the cooling fans operate correctly.
Common symptoms
- Illuminated Check Engine Light
- Hard starting, especially when the engine is already warm
- Poor fuel economy due to the engine running excessively rich
- Rough idle or hesitation during acceleration
- Cooling fans running constantly or failing to turn on when needed
Likely causes
- Faulty Engine Coolant Temperature (ECT) sensor
- Disconnected or loose ECT sensor connector
- Broken or damaged wiring in the ECT sensor circuit (open circuit)
- Corrosion in the ECT sensor connector terminals
- Low engine coolant level causing air pockets near the sensor
- Faulty Engine Control Module (ECM) (rare)
What to check before replacing parts
Visually inspect the ECT sensor, wiring, and connector
Look for: Look for disconnected plugs, broken wires, frayed insulation, or severe corrosion at the sensor terminals that could cause an open circuit.
Check the engine coolant level in the radiator and reservoir
Look for: Look for low coolant levels that could expose the sensor to air instead of liquid, though this is less likely to cause a high circuit code than an electrical fault.
Monitor live data using an OBD-II scan tool
Look for: Look for the ECT reading on the live data stream; a reading of exactly -40°F (-40°C) strongly confirms an open circuit or a completely failed sensor.
Test the ECT sensor resistance using a digital multimeter
Look for: Look for infinite resistance (OL) or readings that are far outside the manufacturer's specified ohms for the current ambient engine temperature.
Check for a 5-volt reference and ground at the sensor connector
Look for: Look for approximately 5 volts on the reference wire and a solid ground on the return wire with the key on; if either is missing, the wiring to the ECM is compromised.
This page explains the generic P0118 definition and is general reference information, not a diagnosis for your specific vehicle. Actual causes, severity, and repairs vary by make, model, engine, and mileage — always confirm with a scan tool and a qualified technician before replacing parts.
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