My Ac rarely reaches the temp set on the themometer. It runs and runs and won't shut off. I have to push it up for it to shut off. This doesn't happen all the time, but a lot. It hasn't worked right since we had the AC put in. The man that installed it never could find the problem. At first it didn't get cold enough in the house. Then they put freon in it and it did. It would get too cold but the themometer hadn't moved much. They replaced the themometer but it's still not right. I see fire fly out of it when it shuts off in the winter after dark. What is that?
Make sure that the hole where the wires come through the wall into the thermostat are caulked or use DUM DUM (that is what it is called in my area). This prevents the warm air in the wall cavity from affecting the thermostat. In many cases when the A/C unit is running properly but the unit never shuts off it is because of this warm wall air situation. The installers usually punch a good size hole in the wall so the wires will pass into the thermostat easily but the problem with that is if ANY space is left around the wire and the hole is the wall it has a bad effect on the thermostat reading and it is always sensing the the warm wall air so it thinks the A/C needs to keep running.
You may check with a different local ON SITE HVAC person to be sure the A/C unit is operating properly to solve any other problems. Good luck and keep cool.