If your microwave is not working at all, check:
1 - Fuses
When your microwave oven doesn't work it all, it is usually due the internal fuse (thermal fuse) being blown. This fuse protects both the oven and you. Only a qualified service technician can replace the fuse.
2 - Door switch
If the fuse isn't the problem, check the door hooks and the door-release button. When these are broken, the door switch doesn't activate, so the oven can't work.
If there's no heat, but the oven sounds normal, either one of the internal door switches, the high voltage circuitry, or the circuit board may be the problem. Contact a qualified appliance repair technician to repair the problem.
If you are having a problem adjusting the power level, there's either a problem on one of the circuit boards or a defective triac (a type of electronic voltage control). Have a qualified appliance repair technician diagnose and repair the problem.
Inspect the plastic drive coupling that the tray sits on, as well as the ring-and-roller assembly, to make sure that they're working properly. The carousel is usually driven by a small motor which is separate from the heating function of the microwave, but because the microwave cabinet needs to be opened to repair it, we suggest you contact a qualified repair technician.
Over-the-range microwave ovens are often plagued by intermittent problems. Moisture from the range/stove can enter the microwave cabinet and collect on the circuitry. The problem is usually with one or two simple connections that may need tightening or soldering. In order to repair these components, the microwave cabinet needs to be opened, so we recommend that you contact a qualified repair technician.
Sometimes the display may flicker mildly but won't affect the way the microwave works. If this is the case you don't need to worry about it. If the flickering is severe and accompanies cooking problems, you will need to contact a qualified appliance repair technician.
If you hear a loud buzz, but the microwave is not heating, the power diode, high-voltage capacitor, or magnetron may be defective. You should contact a qualified appliance repair technician to determine the exact cause problem.
The door being slammed may have blown a fuse. See It doesn't work at all.
When you see arcing inside your microwave, it's because of food or liquid left in the cavity or on the wave-guide cover:
1 - If the wave guide has any burn spots, replace it and clean the unit well.
2 - If the cavity has any burn spots, spot-sand it smooth and touch it up with microwave interior paint.
Note… If there are any burn holes in the cavity, don't use the unit-seek help from a qualified appliance repair technician.