In this scenario lets start with your thermostat and work back to where the problem lies in your system. With most types you just pull it straight out but check your manufacturers.

One Of Two Nest Thermostats Won T Run Fan Only Fan Runs With
Thermostat won t work. Your thermostat may just not be connected right or at all. Make sure the circuit breaker switches for the furnace are on. Before you start troubleshooting. What are you experiencing. If your air conditioning unit wont come on at all then you should check the thermostat. 3 use a soft brush or vacuum with a brush attachment to gently and carefully remove dust and dirt.
Turn off the power at the wall before doing any investigation. If the cooling still doesnt come on but has worked in the past call a professional hvac contractor to assist you. Thermostat does not control temperature 1 turn the power off to the heating system. Try turning the power on again. If your thermostats wi fi isnt working the solution depends on what youre experiencing. If the thermostat indicates a call for cool verify that the furnace door is securely closed and that the power switch on the furnace is set to on.
Firstly ensure that there is power to the thermostat both by checking the wiring supply to it and also by replacing the batteries. New thermostat new installation this is for people who recently bought a new thermostat and are installing it for the first time. Continue to the new thermostat new installation section. 2 remove the thermostats cover. Check the power source. Here are four basic things to look at if your furnace isnt responding to your thermostat settings.
If theres a power issue your thermostats battery will drain and it will turn off wi fi the display and other features to preserve battery life.