Kenmore microwave oven model # 721 63684300 built 5/2003. Does not heat, after 25-30 sec. F9 appears on keypad. Everything else works---timer, clock, lights, horizontal turnstyle.

This indicates a bad door switch, or the keypad control board is not working properly.

That sounds like a problem on the invertor board. Unfortunately there is not a lot to be done on the board without a service manual and the needed experience.
Hope that helps.

Most likely you have a magnetron problem. Easy to replace (DYI) but make sure the high voltage capacitor is fully discharged first against the case.

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My Kenmore Elite Microwave Model No. 721.64683300 does not seem to be heating well and will shut off and flash an F9 code. What does the F9 code mean

This is usually inverter trouble, but it may be the magnetron, controller, wiring, etc.

There should be a "mini-manual" (tech sheet) hidden inside the unit behind the control panel or hidden on the left side behind the grille, which is very helpful when troubleshooting, testing, and locating components.

To find helpful exploded view diagrams and get part numbers, you can just enter your full model number at the Sears parts site.

We're happy to help you with free advice and we'd appreciate your thoughtful rating of our answer.

F9 indicates a problem with the door latch.  There is a safety switch that will not allow the microwave to operate if it detects the door is not closed properly.  Even though physically it may seem the door is shut, the safety switch is not relaying the message to the onboard computer.  You need another door latch switch.

This is a sure sign that the MAGNATRON has burnt out and needs replaced! (If the turntable works and the control panal is still working)! The MAGNATRON is what cooks the food)
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Seems like I answered this, but maybe it's a different poster.

The 721 model code means it's an LG/Goldstar-made oven and F9 indicates a problem with the inverter or magnetron.

Due to the lethal voltages involved, I do not recommend that a person without training and experience ever work on these models.

I would suggest you contact Sears for service.

We're happy to help you and we'd appreciate your thoughtful rating of our answer.

built in 3/94 is it worth fixing how much for magetron