Why does the thermometer display a 'minus' value?

A sensor has deteriorated and is on the brink of cut, or it is attached in the incorrect direction.

A thermometer displays burnout (minus) alarm, when a sensor has deteriorated and is on the brink of cut, or is attached in the incorrect direction.

Troubleshooting

In case a sensor has deteriorated, replace the sensor with a new one in the correct direction.

Attach a sensor to the terminal of thermometer, matching the colors (red and blue).