When a thermostat's backup batteries die completely or suffer from low voltage, the device will shut down its display screen, drop offline from Wi-Fi, and stop communicating with your HVAC system, disabling your heating and cooling control. For standard digital models, replacing the drained batteries with fresh alkaline cells restores immediate operation. For hardwired smart models, low battery warnings indicate a missing or faulty C-wire power supply. Verification: Observe the display screen light up and resume normal operation immediately after replacing or charging the batteries.
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