P0118 — Engine coolant temperature circuit high input

P0118 indicates high electrical input in the engine-coolant-temperature (ECT) sensor circuit, NOT necessarily hot coolant. In a common NTC circuit, an open connection can produce an implausibly cold inferred temperature. Verify the exact engine's wiring diagram and sensor characteristic before interpretation or parts replacement.

Affected system: Engine temperature

Symptoms

• Check-engine light and implausible/unavailable ECT live-data reading.
• On some vehicles: difficult starting, raised idle or higher consumption from incorrect temperature-based fueling.
• Temperature indication and fan operation may enter fallback mode depending on vehicle strategy.

Possible causes

• Disconnected plug, corrosion or open signal/return wiring.
• Internally open or incorrect ECT thermistor, after confirming sensor type.
• Short to reference/supply or controller input fault only after sensor and wiring tests.

What to check

• 1. Record P0118, freeze-frame and related temperature or supply DTCs; note recent work on coolant components and wiring.
• 2. With the engine fully cold compare ECT live data to ambient temperature and IAT if available. Implausibly cold or missing ECT points to a signal issue, not proof that coolant is physically cold.
• 3. Identify the correct sensor using the engine diagram: some vehicles use multiple senders or a separate gauge circuit. Inspect terminals, moisture, connection and chafed wires.
• 4. Confirm sensor type and specifications. For a verified NTC sensor, compare its DISCONNECTED resistance at the measured temperature with the OEM table; do not apply one universal resistance figure.
• 5. Use the service procedure to test signal/reference voltage and return. High input with a good sensor calls for open-circuit or short-to-supply checks; never apply voltage to unidentified pins.
• 6. De-energize/isolate modules as required and check wire continuity and insulation. Readings changing during gentle loom movement support intermittent contact; do not ohm-test a powered ECU circuit.
• 7. Repair only the proven defect, clear DTCs and verify smoothly changing ECT from cold to warm. Separately assess genuine overheating: stop the engine safely for steam or severe temperature warnings and never open a hot cooling system.

Also written as: P0118-00, P011800, PO118 (some scan tools add the fault-type byte, e.g. -00).