Typically, engine operating temperature is around 90 degrees Celsius. However, there may be several temperature sensors, each of which can read the temperature at a different location. Most likely, the gauge is using a different temperature sensor than the ECU is using. Different locations may have different temperatures.
- The ECU temperature sensor is reading the temperature at a different point than the gauge on the dashboard and thus the temperature at the different point differs (most likely explanation as the gauge seats on the center)
- ECU temperature sensor is faulty and reports incorrect readings
- Thermostat is old and opens at a different temperature than a new thermostat opens at, but if the gauge is fine, it would require an equal and opposite error to seem fine