Air Conditioning - AC not AC-ing, Heater not heating

Set programmable thermostat for lower temp on central AC. Compressor did not engage.
Problem same few days ago, but AC kicked in a bitlater.
Tried overide at thermostat, nada.
Late last year had svc man out; no heat. He replaced something in switching in attic unit. Heat worked. Used AC few times this year, now problem seems to be occuring again. Svc man charged $150 to include whatever part he replaced.
Thoughts?
Thanks in advnce
Sarge

If you dont know how to check the unit out with a meter or what he put in new the last time. Id say best to call him back. It might cost less in the long run.
You could try at the tstat and jump R to G see if you get inside fan.

Jump R to Y the outside AC should come on. If they come on that way can be the tstat. Does the tstat have batt. in it??. There are so many IF'S IF'S for why it dont run

I am in process of trying to contact him.
His bill/invoice only shows a part number for whatver he installed
I will try the terminal jump tomorrow when it is daylight,and replacing the battery.
Thanks
Sarge

my thoughts are that if it is that infrequent, change the batteries.
or if there are no batteries get a new t- stat.

Batteries OK, replaced anyway. That seems only to activate display on t'sat
Tried jumping R-G R-Y...nada
Tech who did original work still has not called back. He replaced some type of unit in main located in attic, last time. Number he put on invoice means nothing to a local parts store. He thinks it is the inventory control number for the fellow who installed it.
Will see if I can get a tech who does this for apartment complexes to take a look. He comes highly recommended.
Without knowing what is really going on in the main unit, or what the various components do, or how to check them, I am hesitant about charging my but
up the stairs and poking around.
Thanks
Sarge

You're right, call someone else. AND don't get into something your trained to do or don't have at least a working knowlege of. Mainly electricity.

Chris

He probably changed the transformer or the circuit breaker in the air handler.