Engine Coolant Temperature Circuit High Input
The ECU detects the coolant temperature sensor is reading hotter than physically possible, like a thermometer stuck at 212°F when the engine just started. This usually means the sensor or its wiring is faulty, not that your engine is actually overheating.
The ECU monitors voltage from the Coolant Temperature Sensor (CTS), which decreases as coolant heats up. When the sensor reads abnormally high voltage—typically above 4.8V—the ECU interprets this as an impossible coolant temperature and triggers P0118. The circuit is tested during both key-on and running conditions.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| CTS Voltage | 0.2V to 4.8V (−40°C to 130°C) | >4.8V (temperature reading exceeds max safe limit) |
| Coolant Temp Reading | −40°C to 130°C | >130°C continuously at startup/idle |
Code P0118 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P0118 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.