When the display shows -99, this is an indication that there’s an error reading the temperature sensor.
Verify the configuration screen didn’t get changed. Specifically, look at the “TEMP SOURCE” settings. On very early model Waiter ECCs that have the FRONT and REAR sensors plugged into the control module, the “TEMP SOURCE” should be set at “1 0 0”. All newer Plug and Play versions that have the temp sensors plugged into the back of the touchscreen, the “TEMP SOURCE” should be set at “1 4 4”.
If its only one of the temperatures that display the -99, consider a bad sensor or broken wire. If its the FRONT or REAR, swap the plugs on the back of the touchscreen to see if the problem changes from front to rear.
If its the outside temperature, try resetting the three pin plug on the control module, visually inspect the sensor prob for damage.