P0118 OBD2 Code Guide
P0118 is a sensors diagnostic trouble code. Use this hub to understand the plain meaning, the first checks to run, and which vehicle-specific guides may be more useful before replacing parts.
Engine Coolant Temperature (ECT) Circuit High Input
The P0118 code signifies that the Engine Control Module (ECM) or Powertrain Control Module (PCM) has detected a voltage reading from the Engine Coolant Temperature (ECT) sensor that is higher than the specified range. A high voltage reading from a thermistor-type sensor (which most ECT sensors are) indicates an extremely low or even open circuit temperature, often interpreted by the ECM as -40°F/-40°C or colder. This discrepancy can lead to the ECM believing the engine is much colder than it actually is, affecting fuel delivery, ignition timing, and cooling fan operation, potentially causing rich fuel conditions, poor fuel economy, hard starting, or overheating if the fans don't engage correctly.
Code verification standard
P0118 is published with SAE J2012 / ISO 15031-6 code structure plus OBD2 P0118 diagnostic context.
First checks
- - Read all stored and pending codes before clearing anything.
- - Save freeze-frame data so you know speed, load and temperature when the code set.
- - Inspect wiring, connectors, fuses, hoses and obvious leaks tied to the named system.
- - Compare live data against realistic values before buying sensors or modules.
- - Clear the code only after repair and complete a drive cycle to confirm it does not return.
P0118 diagnostic path
- 1Visual Inspection: Check the ECT sensor connector and wiring for corrosion, damage, or loose connections. Inspect the sensor itself for physical damage.
- 2Scan Tool Data Analysis: Monitor live ECT sensor data with a scan tool. Compare the reported temperature to ambient temperature when the engine is cold. If it reads extremely low (-40°F/-40°C) or stuck, it confirms the issue. Also, check for other related codes.
- 3ECT Sensor Resistance Test: Disconnect the ECT sensor and measure its resistance across its terminals using a multimeter. Compare the reading to the manufacturer's specifications at various temperatures (e.g., cold engine, warm engine). An open circuit (infinite resistance) or significantly high resistance indicates a faulty sensor.
- 4Circuit Voltage/Continuity Test: With the sensor disconnected, check for reference voltage (typically 5V) at one terminal of the connector and a good ground at the other. Test for continuity in the signal wire back to the ECM/PCM. A missing reference voltage or ground, or a short/open in the wiring, could be the cause.
- 5ECM/PCM Pin Test (Advanced): If all sensor and wiring tests pass, perform a resistance test directly at the ECM/PCM connector pins for the ECT circuit to rule out internal wiring issues or a faulty ECM/PCM input.
Verify before repair
- - Replace the ECT Sensor: This is the most common fix if the sensor itself is faulty (open circuit, incorrect resistance).
- - Repair/Replace Damaged Wiring or Connector: Address any corroded, frayed, or broken wires/connectors in the ECT sensor circuit.
- - Clean Corrosion: If corrosion is present on the sensor terminals or connector pins, clean them thoroughly.
- - Replace the ECM/PCM: This is a rare fix and only considered after all other possibilities have been exhausted and confirmed by professional diagnosis.
Vehicle-specific pages to check next
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