A safety note up front: microwave ovens contain a high-voltage capacitor that can store a lethal charge for days after being unplugged. Do not open the cabinet unless you know how to safely discharge it with an insulated screwdriver across both terminals to ground. Most microwave problems can be diagnosed without ever opening the cabinet — and many are not worth repairing on a unit that costs less than $100 to replace.
Magnetron failure, high-voltage diode shorted, capacitor failed, or door interlock not closing fully. Symptom-only diagnosis: if the unit runs and the turntable spins but cold water in a mug stays cold after sixty seconds, you have a high-voltage failure. On a unit out of warranty and more than five years old, replacement is almost always the right call.
Stop immediately. Almost always one of three things: metal in the cavity (twist ties, foil, decorated plates), a burned wave guide cover (the small mica panel on the inside wall — replace it), or carbonized food splatter inside the cavity. Clean the cavity, replace the wave guide cover, and run a short test with a mug of water.
Door interlocks (three or four switches in the door latch — if any one fails, the unit refuses to run), failed touch panel, blown line fuse, or failed control board. Touch panels on over-the-range models often fail from steam exposure.
Display itself has failed, ribbon cable disconnected, or membrane keypad failed. On older units with vacuum fluorescent displays, dimming is age-related and usually not worth fixing.
Magnetron failing under load. Can also be a failed high-voltage transformer. Either way, the high-voltage section needs service or replacement.
Roller ring under the glass tray has dropped out of position, drive coupler is missing, or turntable motor has failed. The motor lives under the cavity floor; replacement is straightforward.
Failed fan motor capacitor, or the speed switch on the control panel. The fan is independent of the cooking circuit.
Bulb (most over-the-range models use halogen capsule bulbs; some need to be ordered from the manufacturer), failed light socket, or burned-out interior bulb that's accessible only by removing the cabinet — at which point the safety warning above applies.
Any high-voltage symptom (no heat, humming, sparks from the back) on a unit you value is technician work — the capacitor will kill you if you misjudge it. For most countertop units priced under $200 new, replacement beats repair.
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