Ac problem with the Honeywell Chronotherm III Therm

I just got a Chronotherm III to replace my old Honeywell dial therm. When I installed it I get the -AC message. Witch in using the groups I found out means no Ac power. What is wierd is when I turn off auto and switch to the fan, it comes on.
Any Ideals, Any would help
THank You
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Easy enough. You replaced a T87F that did not have a common lead. Ten to one, the wire that is missing, or isnt hooked up at the thermostat, or the unit since it wasnt needed, was the C line. You still have power to the stat, the stat just has no way to know it.
The reason your fan comes on, is that like your old stat, the auto-on switch is nothing but that...a switch. You are completing the circuit to the fan relay.

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