P0118: Engine Coolant Temperature (ECT) Circuit High

P0118 means the engine coolant temperature (ECT) sensor circuit voltage is too high, which the ECU reads as an implausibly low temperature. Because the sensor is a thermistor whose resistance rises as it cools, an open circuit or a disconnected sensor produces a high voltage that mimics extreme cold. The ECU then over-fuels for a perpetual cold start. Repair typically costs 40 to 180 EUR for a sensor or wiring fix.

P0118 is a cooling system-related diagnostic trouble code classified as moderate severity. When your vehicle's ECU detects this condition, it stores P0118 and illuminates the check engine light.

If your vehicle has triggered P0118, you may notice one or more of these symptoms:

• Check engine light illuminated
• Temperature gauge reading cold or pinned low
• Rich running, black smoke and a strong fuel smell
• Poor fuel economy
• Cooling fan not engaging when it should
• Rough idle when the engine is actually warm

Here are the most common causes of P0118, ranked by how likely they are to be the culprit:

Generally yes, for short-term driving. Usually safe to drive short distances. P0118 means the coolant sensor signal voltage is too high, which the ECU reads as an implausibly cold engine (an open or high-resistance circuit mimics extreme cold). This richens cold-start fuelling, can keep the cooling fan off when it is needed, and worsens fuel economy. Because the gauge may show a false low reading, confirm the real temperature in live data and fix the fault promptly so you do not miss actual overheating.

Follow these steps to pinpoint the root cause of P0118:

1. Read the code and watch the live coolant temperature PID with Skanyx. P0118 typically shows the minimum value (often around -40 C) even on a warm engine, which is a clear sign of an open circuit rather than a real cold reading
2. Touch the upper radiator hose to gauge actual temperature; if the engine is hot but the PID reads -40 C, the circuit is open
3. Check the sensor connector first, since a disconnected or loose plug is a frequent and easy fix; reseat it and recheck the live reading
4. Inspect the connector terminals and harness for corrosion, spread pins, or a broken signal wire causing an open circuit
5. Measure the sensor resistance with a multimeter; an infinite or very high reading confirms an internally open thermistor
6. If the sensor and connector are good, test the signal and ground wires for continuity back to the ECU

Repair costs for P0118 vary depending on the root cause and your vehicle. Here are typical estimates:

P0118 behaves differently depending on your vehicle. Select your car below for model-specific causes, known issues, and adjusted cost estimates:

These codes are often seen alongside P0118 or indicate related issues: