What should I check before I call?
Thermostat, breakers, filter — in that order.Make sure the thermostat is set to the right mode (cool or heat, not fan or off) and to a temperature that should make the system run. Check the breakers for both the outdoor unit and the indoor unit — there are usually two. Look at the filter; if it is grey and matted, change it. If all three are fine and the system still is not working, call.