AC won't turn on unless fan switched turned to ON from AUTO
 

I currently rent out a home I own in IL and live in IN. The renter called and stated that they had to switch the thermostat from auto to on to cool down the house. Could this be a battery that is dying and not tripping a switch to turn the fan on? I couldn't tell you the last time I changed that battery, its probably 3-4 years old at least. That is just the first thing the came to mind.

Any thoughts?

Yep definately could be. I ran across one very similar the other day. It's a bummer that a couple of aa batteries can hold the power of life in a whole hvac system.

I'm thinking that the transformer would continue to power the thermostat and the batteries are just to maintain the programming in memory in the event of a power outage.
Could be that the thermostat is bad but, only troubleshooting will reveal the problem.

The transformer can't power a thermostat unless it is a five wire configuration where a common wire has been brought from the transformer and connected to a "c" lug. In this case batteries would be optional and useless. Setting are stored in non volatile memory that needs no power.
In a normal 4 wire config, the batteries run the thermostat. Red is power and every other wire an extension of that as the thermostat calls for it. Below is from a honeywell non programmable thermostat manual.


1. Replace the batteries as soon as Replace
Battery flashes in the display. The Replace Battery
indicator flashes in the display two months before
the batteries run down completely.
2. Always use fresh AAA alkaline batteries. Nonalkaline
batteries do not last as long and can leak,
causing damage to the thermostat.
3. Although the thermostat has a Replace Battery
indicator, replace the batteries once per year to
prevent the thermostat and heating/cooling system
from shutting down due to a lack of battery power.
NOTE: Replacing the batteries once a year also helps
prevent battery leakage that can damage the
thermostat.
4. As a precaution, replace the batteries when leaving
your home for more than a month to prevent your
heating/cooling system from shutting down if the
batteries run down completely.