What does error code F9 mean on a Kenmore microwave/hood model 721.63689300?

I found that the output relay on the control board was not conducting consistently. It had between 1 and 20 ohms when closed. In my opinion, it undersized. It is not user changable. I got a much larger relay, removed the original one and mounted it in the bottom of the cabinet. The oven still makes a funny 8 khz squawk at the beginning of cooking, but the f9 error code has not come again. We love the oven but would expect longer than 5 years before a major expense. We have a 23 year old Samsung that I have never even had the cover off!

The F9 error code indicates an inverter failure. This can sometimes by caused by a power surge or glitch through the electrical lines. You can sometimes clear this fault by completely unplugging the microwave for 1 minute. Plug it back in and see if the fault code returns. If it does, then you have an internal component failure that will need to be serviced by a technician. Microwaves must be serviced by technicians due to the inherent dangers involved in repairing these appliances. Internal capacitors can retain a lethal electrical charge even though the unit is completely unplugged. A microwave radiation leakage test must be performed on the unit following any internal component repair.

I continued to have the F9 error, so I replaced the inverter board. I reinstalled the oven and it worked for a few days. Even though the test instructions inside the oven stated the test for the magnetron stated the resistance of the coil was less than 1 ohm and infinite to ground and the magnetron tested ok, I replaced it anyway. It now works ok. I have now spent more than what a equivalent counter top model would have cost. Boy, do I regret getting an over the range microwave!

Just to expand on the f-9 code: it does not necessarily indicate a inverter control board problem. LG's service manual troubleshooting section suggests the f-9 code could be due to line voltage, magnetron, inverter module, "Relay2", or a loose connection between door/magnetron/control pcb. Other indications might help narrow down these choices before having to pull out your multimeter and crack open the microwave case; for example, if the microwave works, but heats slowly, then the troubleshooting manual narrows your choices down to magnetron or line voltage problems, and pretty much rules out the other possiblities.