Case 1: The probable cause is that the sensor wire is almost cut due to deterioration. When the sensor has deteriorated, the unit may show the burnout indication (minus-sign indication). Replace the sensor with a new one.
* Photo of the FG-100 discontined.
Case 2: It is suspected that the sensor is mounted in the wrong orientation. Be sure to connect the sensor tubes to the terminals marked with the same colors (red/blue) on the unit respectively.