How do you fix a thermostat that is not communicating?

Fixing a thermostat that is not communicating depends on whether it is a Wi-Fi smart thermostat or a conventional low-voltage system losing communication with the HVAC equipment. Start by checking power, as 90 percent of communication failures are power related. Verify the thermostat display is on, check the breaker for the air handler or furnace, and check if the C-wire is connected and providing 24 volts. For smart thermostats that lost Wi-Fi communication, restart the thermostat by removing it from the wall plate for 30 seconds, restart your router, and ensure it is within range and on a 2.4 GHz network, as many thermostats do not support 5 GHz. Check the app for signal strength and reconnect the device if needed. For systems showing a communication error between thermostat and control board, such as E1 or COMM errors, turn off power at the breaker, remove the thermostat, inspect the wiring for loose connections, corrosion, or a short, especially the R, C, W, Y, and G wires. If you have a communicating system like Carrier Infinity or Trane ComfortLink, check for a wiring error or mismatched equipment as these systems use ABCD or data wires that are sensitive to polarity. If none of these steps restore communication, the thermostat, control board, or wiring may be faulty and require a multimeter test or professional HVAC technician.