Press both the up and down arrows simultaneously for 3 seconds and let go. You will see F for Fahrenheit or C for Celcius. Press the light button or wait 5 seconds and the differential will show. Press the light button or wait 5 seconds and the filter monitor will show. It is adjustable with the up and down arrows from off-9900 hours in 100 hour increments. When finished leave the thermostat alone and it will return to the main screen.