Performed furnace diagnostic 7 yr York split closet. Found unit turns on and runs, then turns off, Trips safety switch, four flashes, lack of airflow found filter in stamp face grill very very dirty. No filter in blower compartment. Filter for that would need to be a 18-25-1. Or we could change the stamp face grill to a return air filter grill. Or we could get a custom washable filter made. ALso recommend maintenance plan, to look thru system and make sure nothing else is going on. Took filter out, they painted over it in places. And tested system, system warmed up house no issues, went from 65 to 69