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I got some information from Honeywell which was helpful.This thermostat requires 2 AA, alkaline, non rechargeable batteries. Make sure 2 of AA, alkaline, non rechargeable batteries are correctly installed.Check the contacts in the battery holder for any signs of corrosion or discoloration. Corrosion on the battery terminals can cause the thermostat not to register new batteries. If there is oxidation or corrosion on the battery terminals, it needs to be cleaned off. Do not use anything abrasive to clean the contacts as this may damage the thermostat. It is advisable to use an electronic contact cleaner to clean the contacts. Spray electrical contact cleaner onto a soft cloth and wipe down the battery contacts with the soft cloth. Electrical contact cleaner is available through electronic stores such as Radio Shack.If the thermostat still shows Bat lo on the display, we recommend resetting the microprocessor of the thermostat. This can be done by placing the system switch in the off position, removing the batteries, and putting them in backwards for 5 to 10 seconds. Reinstall the batteries in the correct polarity. The thermostat's display should be flashing 1:00 PM at this point. The program and current time and day will need to be reset.If these steps do not help, thermostat will need to be replaced
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