1. Failed magnetron — the #1 cause
What it is: The magnetron is the vacuum-tube that generates the microwave radiation. They typically run 8–12 years on countertop and over-the-range (OTR) models, less on heavily-used units. Symptom: microwave runs the full timer, fan and turntable spin, light is on — water comes out cold. A magnetron going out also often hums louder than normal or buzzes during cook. GE / Whirlpool / KitchenAid / Samsung OTR units are the most-frequent service.
Fix: Verify with a 1-minute water test, discharge HV capacitor, test magnetron filament continuity and shorts to ground, replace OEM magnetron with new HV diode and thermal cutout in the same visit.
Typical all-in: $345–$465 all-in
2. Blown high-voltage (HV) diode
What it is: The HV diode (sometimes called the rectifier) doubles the voltage going into the magnetron. When it shorts, the magnetron has voltage but can't fire — no heat. When it opens, the HV fuse blows. Symptom: runs without heat OR runs for 2 seconds then blows the internal fuse. Common across every brand at year 6+.
Fix: Discharge HV capacitor (this is the dangerous step — leave it to a tech), test diode in both polarities, replace OEM diode. Always paired with HV fuse and capacitor inspection.
Typical all-in: $265–$385 all-in
3. Failed door interlock switches (very common — and a safety system)
What it is: Every microwave has 3 or 4 interlock switches stacked at the door latch — they kill the magnetron the instant the door cracks open. When one fails (usually the monitor switch or the primary), the unit lights up and the fan runs but the magnetron never fires. Symptom: runs, doesn't heat; sometimes a hum, sometimes silence. Often follows a hard door slam or a warped frame on OTR units.
Fix: Test each interlock for correct make/break sequence, replace OEM switch set (always replace as a kit — they wear together). Verify door alignment and latch hook condition.
Typical all-in: $245–$345 all-in
4. Blown HV fuse (downstream of a real failure)
What it is: If the HV fuse is blown, something blew it — a shorted diode, a shorted capacitor, a shorted magnetron, or a failed monitor switch closing the kill-circuit. Symptom: completely dead, or runs the timer with no sounds at all from the magnetron section. Never replace just the fuse — find why it blew.
Fix: Discharge capacitor, test diode / capacitor / magnetron / monitor switch in sequence, replace failed part(s) AND the HV fuse, full HV system test before closing.
Typical all-in: $245–$385 all-in
5. Failed high-voltage capacitor
What it is: The capacitor stores the HV charge feeding the magnetron. When it opens, the magnetron loses its voltage doubler and won't heat. When it shorts, it blows the HV fuse instantly. Symptom: runs without heat, or trips the breaker. We find this on roughly 1 in 10 'not heating' calls, usually older Whirlpool / Frigidaire / Panasonic countertop units.
Fix: Discharge, test for capacitance and short-to-ground, replace OEM capacitor. Always paired with HV fuse and diode replacement — the part costs $20, the labor is the discharge / test routine.
Typical all-in: $265–$365 all-in
6. Tripped thermal cutout (cavity / magnetron over-temp)
What it is: Every microwave has 1–2 thermal cutouts that open if the cavity or magnetron overheats — usually from running an empty cavity or from a blocked exhaust vent on OTR units (grease-clogged charcoal filter). Symptom: heats fine when cold, quits heating mid-cycle and stays cold even after cooling.
Fix: Test thermal cutouts for continuity at room temp, replace OEM as found. Clean / replace OTR charcoal filter and grease filter. Verify cavity isn't being used empty.
Typical all-in: $225–$345 all-in
7. Failed main control board or HV transformer (rare — last suspect)
What it is: Past everything above, the main control board can fail to send the HV-call signal (about 3% of calls), or the HV transformer itself can fail open (rarer still — usually after a power surge). Symptom: confirmed good interlocks, diode, capacitor, magnetron, fuse — and still no heat. On a unit past year 8, this is the cutoff for repair-vs-replace.
Fix: Verify HV transformer primary / secondary voltages in service mode, replace OEM control or transformer as found. We quote the all-in number and walk through repair-vs-replace honestly.
Typical all-in: $425–$585 all-in
