I used this excellent guide to diagnose the problem: h You need a safety Torx bit to open the case, and carefully follow instructions to discharge the high voltage capacitor! Following the youtube instructions and a commenter's advice to use a 9V battery in series with the multimeter instead of the cumbersome 12 V car battery method, I found that the diode was defective (low resistance). This was easy to replace, but the microwave still did not heat. The diagnostics video cautioned that the tests used do not identify all bad magnetrons. The antenna on mine was discolored, so ordered a replacement, and this fixed the problem.
Everything Worked Fine But It Stopped Heating The Food.
Take the four saftey torx screws off the back of microwave that hold the side panels on. There are no screws in the front. Just pick up the side panels a little and slide towards the back of microwave and it will come off. The magnetron is visible behind control panel. Unplug the magetron, take a 3/8 nut driver , remove the one bolt that holds it on , remove and replace with the new one. Plug the wire (it will only go one way) back up and replace cover.
I just took off the cover, located the magnetron, replaced it and slapped everything back together and since then everything's been working pretty good.