Symptoms
- Screen frozen, blank, or unresponsive to touch/buttons
- Changing the setpoint doesn't start the AC
- Display may flicker or show a low-battery warning
Likely causes
- CommonDead or weak batteriesMost thermostats use batteries for the display and memory even when also wired into the system.
- CommonTripped breaker cutting power to the thermostatSome thermostats are powered through the air handler's low-voltage transformer.
- PossibleLoose or corroded wiring at the thermostat baseA wire can work loose from vibration or a past thermostat swap.
- PossibleSmart thermostat software glitch or lost Wi-Fi connectionSmart thermostats occasionally need a reboot like any small computer.
- Less commonFailed transformer inside the air handlerIf the transformer fails, the thermostat gets no power at all.
What to do
- Replace the batteries even if the display still shows something.
- Check the breaker for the air handler hasn't tripped.
- Gently remove the thermostat faceplate and check that wires are firmly seated in their labeled terminals.
- For a smart thermostat, try a manual restart per the manufacturer's instructions.
- If the screen is completely dead with fresh batteries and a good breaker, the transformer may have failed.
- Avoid rewiring the thermostat yourself unless you've labeled and photographed the original wires first.
Tools and parts
- New batteries
- Small screwdriver
Thermostat batteriesSearch: “AA or AAA batteries thermostat”
When to call a pro
Call an HVAC technician if fresh batteries and a breaker check don't restore the display; the transformer may need replacement.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.