A microwave that runs but doesn't heat has a failure in the high-voltage circuit — magnetron, diode, capacitor, or transformer. Unlike most appliance repairs, this one is NOT a DIY job: the HV capacitor stores a lethal charge even after the microwave is unplugged. Here's what's actually wrong, and when it's worth fixing vs. replacing.
Most microwave symptoms get diagnosed and fixed on the first visit. Flat $129 diagnostic — fully waived when you approve the repair.
Step-by-step
Place a cup of water inside, run for 1 minute on high. Pull it out — should be hot. Cold water with a normally-running microwave (turntable spinning, light on, fan running) confirms the heating circuit has failed.
The door has 3 switches that all have to engage to allow heat. A misaligned door — even slightly — disengages one of them. Press the door firmly closed and try again. If it heats, the door alignment or switches need adjustment.
A failing magnetron often makes a louder-than-normal hum or buzz before going completely silent. A loud pop or burning smell + no heat = magnetron or capacitor failure.
Magnetron + labor: $250–$400. New countertop microwave: $100–$300. New OTR (over-the-range): $400–$700. For countertops, replacement almost always wins. For OTR and built-in, repair often makes sense if the unit is under 5 years old.
DO NOT open the cabinet. The HV capacitor stores up to 4,000V even after the microwave is unplugged — discharging it requires a tool and technique. Microwave repair injuries are uncommon precisely because most people know not to open them. Keep that streak alive.
Stop and call
Safety + model triggers
Microwave repair — high-voltage capacitor stores up to 4,000V even when unplugged.
ALWAYS a tech call. HV capacitor injuries are how people die fixing microwaves. Do not open the cabinet.
Burning smell, smoke, or visible flame inside the cavity.
Magnetron or waveguide failure — unplug at the wall immediately and don't run again.
Microwave runs but trips the GFCI or breaker on heat cycles.
Failing diode or transformer drawing a short — DO NOT keep resetting the breaker.
Over-the-range, built-in trim-kit, or microwave drawer (Sharp, Wolf, Bosch).
Mounting and venting are integrated — pulling the unit wrong damages cabinetry and the OTR fan housing.
Other reasons to call
FAQs
The high-voltage circuit (magnetron, diode, capacitor, or transformer) has failed. The control board, fan, light, and turntable all run from the low-voltage circuit, so the microwave appears 'on' even though the heating circuit is dead.
Countertop microwave: usually no — replacement ($100–$300) costs less than the repair ($250–$400). Over-the-range or built-in microwave: often yes if under 5 years old, since replacement runs $400–$1,200+ plus install.
No. The high-voltage capacitor stores a lethal charge even after the microwave is unplugged. Every year people are seriously injured opening microwaves. This is the one appliance repair where the call is always worth it.
8–10 years for countertop models, 10–15 for over-the-range and built-in. Magnetrons gradually weaken before failing — slow heating is the early warning sign.
Usually a damaged waveguide cover (the small mica panel on the side wall) — cheap fix. Could also be metal stuck to the cavity wall, or a failing stirrer. Stop using until inspected — sparking can damage the magnetron.
Related service
Over-the-range, countertop, built-in trim-kit, and Sharp microwave drawers — we service every kind. Same-day windows across the Twin Cities and Central MN with OEM parts on every truck and a flat $129 diagnostic that's waived when you approve the repair.
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