Different fans. There is your vent fan, and there is the magnetron fan. The magnetron fan blade is the bad one. You can look behind the control panel, remove the vent cover, take out the 1 screw at the top of the control panel. Push up on the panel and it will come off, and you can see the fan there. I replace the fan blades with the control off, kinda tricky though. Or you can remove the M/W remove the outer casing and do it that way. You do not need to replace the magnetron. and yes the fan comes on when you press start. Drop the control panel down and program to cook, and watch the fan blade. Does your M/W vent to the outside of the house, or forward into the kitchen? This will tell you if you need a charcoal filter or not. You only need the filter if the vent exhausts back into the kitchen.