Panasonic 1250 Watt genius microwave- will not cook. message refer to operators manual before use. Have owned for several years, has run afew times after unplugging for a few minutes, then nothing. clock, light works.

 Confirm 110-120 volts AC is entering the primary coil on the high voltage transformer....meter works but can be fooled, I usually rig up a light socket and household light bulb and hook that to the 2 input wires for the transformer and see if the light works or not when you run the microwave....can even set a power level and watch the light go on and off. Discharge the capacitor first before attempting any checks or tests.


Scary but works!....

If ok, put those 2 wires back on the transformer. Discharge the capacitor .
Remove the 2 wires going to the magnetron....run the microwave a few seconds ( 2-3 ) and remove power....again discharge the high voltage capacitor . If you got a good crack&spark when you discharge the cap the second time you are creating high voltage but not using it ( bad magnetron or loose High Voltage wire somewhere )....if you get no crack&spark you are not creating high voltage.
 
How to check a common high voltage diode - click here. Check the diode with one end off/disconnected....should show continuity one way, reverse your meter leads and none the other way.....many meters are not high/good enough and often show no continuity ( infinity ) in both directions. A full continuity reading in both directions  (0 ohms) on the diode may be shorted/bad one!
Many  microwaves however have a defect in the microwave distribution system which causes the magnetron tube to overheat and cut out and/or damage the diode.