I came home and the programmable thermostat says "no ac power". So I check the batteries in the thermostat. Good. Then I check the breakers. Good. Then I check the outside unit. Obviously nothing is going on but I did hit the main contactor and the unit came on. I thought that was interesting. I tried to check the fuses outside, but I'm like the unit would not have come on if they were bad. I'm just wondering if the thermostat is bad. I need help quick!

Usually a no ac signal means that no power is being supplied to the thermost from the inside air handler/furnace.

Check fuses/circuit breakers for the inside unit.
Check the 24 volt transformer that is in the inside unit for 24 volts output.

Report back after testing.

Quote:

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Originally Posted by hvac1000

Usually a no ac signal means that no power is being supplied to the thermost from the inside air handler/furnace.

Check fuses/circuit breakers for the inside unit.
Check the 24 volt transformer that is in the inside unit for 24 volts output.

Report back after testing.

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Ok so I checked the transformer and I got 20v instead of the 24v I expected. I checked the red and white wires coming from the transformer. I checked them at the thermostat and I got .8 volts.

You have a defective wire somewhere not allowing the voltage to pass to the thermostat.
OR it is possible a fuse blew on the circuit board.

OR there is a device like a limit control or another control in the inside unit to not allow the 24 volts to pass to the thermostat.

That seems like a lot of places to check but I am just trying to give you as many options as I can think of.

BTW 20 volts could be OK since I do not know how accurate your meter is.
You might remove all the wiring on one side of the transformer and retest. It could have a small load on it that would drop the voltage.