Jim, I think you have a bad temperature sensor. A minus 8 degree reading is too far out range to be adjusted with adjusting screw inside the sensor.
The best way to check the sensor is to pack it in snow or crushed ice for 15 minutes. After this time you should get a reading of 32 or 33 degrees. If it is no more than 5 degrees off you can calibrate the sensor by turning the tiny screw embedded in the silicone on the back of sensor. Turning the screw clockwise 1 complete turn should lower the reading approximately 2 degrees, and of course, counterclockwise should raise the reading.
