How to Troubleshoot a Honeywell 8400C Thermostat

Art Corvelay

Honeywell is one of the most prevalent thermostat brands. Further, the Honeywell 8400C is a very popular thermostat that allows users to program temperature settings for when your residence is occupied and when your residence is unoccupied. There are some common problems that can arise in the Honeywell 8400C, but many of these problems can be solved with a few troubleshooting steps.

  1. Check that the thermostat is properly mounted and attached to the back plate or wall plate if the display on your thermostat is dim or there is no display at all.

  2. Make sure that a fuse is not blown if your thermostat does not power on or there is no display. To do this, check your circuit breaker for the heating and air conditioning systems.

  3. Make sure that you have the correct timing (AM/PM) if your program is not running correctly. If you have your thermostat set for PM when you meant AM, your program will run on an opposite schedule than you planned.

  4. Turn off the entire system and switch it back on if your heat or air conditioning is not working. Once you have turned it back on, run heat or cool cycle for at least 5 minutes.

  5. Reprogram your Honeywell 8400C for both occupied an unoccupied use if the program is not running correctly. To do this, press "set > day > occupied > up > select temperature".