Today the heating/AC thermostat display started showing only HOLD-AC, even though the settings seems to be correct for calling for AC or heating.
I see this question has been posted before, but I don't find a previous answer. Can someone help please? Thanks.

Does the unit have batterys?

Here is a copy of another post.

I have two Honeywell Chronotherm III thermostats. One upstairs and one downstairs. My downstairs air "handler" is in the garage and my upstairs air "handler" is in the attic. One day my upstairs air conditioner quit on me and my Honeywell Chronotherm III thermostat read "Hold -AC". Let me tell you what this means. This means there is no electricity getting to your air handler. There may be electricity to your handler but not allowing it in. This is what you should do first, identify the breaker, in your electrical box at the home, that feeds the handler and make sure it isn't tripped (Flip the breaker "Off" then back "On") if that was the problem, then your thermostat will read "Hold 78"(or whatever temp you have it set to). Make sure the temp setting, on the thermostat, is lower than actual temperature inside your home. It will then turn on after a couple of minutes.(be patient) If that doesn't do it, which it didn't in my case, the other fix is to go to the air "handler" and make sure the doors are properly affixed to the air "Handler",

The Chronotherm III does have AA batteries in it, and they are o.k.
I had a service man here this morning, and he said the limit switch had a problem and he fixed it. I appreciate your prompt answer!