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THe ac normally means we have lost power from the furnace or air handler.Do you know model on thermostat?
Let's do this. Locate the breaker for the indoor unit.Turn breaker off and back on and see if the AC will go away.If not we will go from here.
Do you have a volt meter to troubleshoot? If so we want to go to thermostat, remove cover exposing wires and check between R and C terminals on thermostat for 24 volts.We will go from here.
Did the AC go away and does thermostat work now?
Ok we will need to get that meter to troubleshoot.
The code is telling us thermostat is not sensing the 24v from furnace.
1.If there is no 24v go to furnace or air handler and open it
2.Look for a small auto type fuse they may have blown.
3.Check for high voltage into and low voltage out of transformer..
You can leave the question open until we have solved the issue.
That s a 3 amp fuse. Tell me the model on your furnace
Meter should be good.Check the 24 volts out of transformer on unit and we will go from there.