Most AC units, whether they’re window units or central systems, will have a number of electrical wires running through them, and oftentimes they’re hidden inside a drywall to keep from being obstructions. Pests like rats, however, can get into these tiny spaces and chew up the wires, electrocuting themselves and cutting off power from and to your AC.
Just as your AC is connected to the main power grid via electrical wires, the thermostat is similarly connected to the AC system. These wires might corrode over time, or they could get dusty and frayed, which could weaken their function and cut off your AC’s temperature detection system. With power activity undetected, you wouldn’t know why your power bills start to balloon.