Why does my thermostat show code 'PF' after running the fan or A/C?
HVAC is losing power.


Cause 1 overflow shutoff switch.
Cause 2 bad transformer.


My first inclination is that you have a condensate shutoff switch activated, due to a full pan due to over condensation and/or a clogged overflow pipe. If you had a really nasty old filter, or no filter for some time, then this is even more likely.