Verify that your heat pump thermostat wiring is properly attached, according to the wiring diagram. “Make sure the wires are tight and all the way in the new thermostat,” Sherman says.

The wiring may also be switched for your fan settings. If you're comfortable doing so, you can try to switch the G wire, which controls the HVAC fan, with the Y wire, which often controls cooling.

2. Pop Off the Faceplate You’ll need to remove the faceplate of your old thermostat to get at the low-voltage wiring and backplate. It should pop right off with a little bit of effort.

Alternatively, you can screenshot your wiring diagram before you disconnect the Nest from the wall. Open the Nest app, tap the Settings cog in the upper-right corner, then select Equipment > Wiring.

Whether the thermostat needs to connect to high-voltage wiring: Some thermostats need to control high-voltage appliances, notably large electric heaters. But the average smart thermostat can't ...