I think the most likely PCB components to fail, (if there is no problem with the interlock switches or bad PCB solder connections), for the symptoms you are describing, would be one, or both of the relays failing. You can either check for DC voltage across the relay coil winding terminals (relay 4906=turntable & relay 4901=fan) by probing them with a VOM, or feel or listen closely to the relays to see if they are clicking on when they should, by putting one end of a long wooden dowel (or other insulated) item between one of the relay covers, and the other end of the dowel against that little nub of your ear, that protrudes out and over the opening to your ear canal, pushing it in to close off the ear canal. Your hearing in that ear will become muffled, but you will clearly hear the relay click on, that the other end of the dowel is in contact with. Although you may hear the relay click on, the relay 110v contacts may still be burnt or otherwise be defective; but you will surely know that the other electronics is triggering, or firing the relays on. I would also carefully check those two relays' solder connections, by resoldering them to be sure. Have you actually applied 110v directly to the fan & turntable motors, (after unplugging them from their PCB connectors), to be sure the motors actually turn, and are not just frozen mechanically by hardened grease or by dirt or other debris somehow? There usually is plenty of greasy grime that gets circulated around inside of a microwave oven by the fan, due to steam coming off of the food or beverage; also collecting dust and dirt. Otherwise, trace and check all wiring for continuity thru all plugs and connectors to the relay contact terminal solder connections.