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You will want to look at the backplate (take thermostat off the subbase) is there a wire connected to C?
Also while you have it off, get the model number. It should start with a T, a CT, or some other letter followed by 4 numbers, a letter and more numbers.
Without a Common (C) hooked up, the thermostat will be primarily powered by the batteries. If you have an extra thermostat wire we can walk you through hooking it to the furnace and thermostat.