F9 on a microwave can be caused from several things, but it all means the same thing–the magnetron is not drawing the amount of power needed. This error will usually happen on LG and Kenmore related microwaves. You could have a faulty magnetron itself, a faulty inverter board, or one of your door switches can just not be activating correctly. You can’t test the magnetron or the inverter board, so the best thing to do is check the door switches. Unplug the unit and remove the control panel. This will give you access to the door switches to the left of the control panel opening. There should be 3 switches which you can remove and make sure they don’t look burnt. Push on the little actuator buttons and see if the switches still click. If so then reinstall the switches and shut the door to make sure the door is activating all the switches when the door shuts. If so, then it’s time to guess which part you want to replace–either the magnetron or the inverter board. The manufacturer will tell you to replace the inverter board first, but it’s really a toss up .