Before anything else, one warning that matters more than every tip below: a microwave stores a dangerous electrical charge inside its cabinet, and that charge can remain after the plug is pulled. The high-voltage capacitor that feeds the magnetron is the reason. Nothing in this article involves taking the outer case off, and nothing should. Everything listed for you to check can be done from the outside or from inside the cooking cavity, with the unit unplugged or the breaker off.
With that settled, most microwave failures fall into a handful of patterns, and the pattern tells you a lot.
It runs, the plate turns, the food stays cold
This is the classic one. The light comes on, the fan blows, the turntable spins, the timer counts down, and the coffee comes out the temperature it went in. Everything that runs on ordinary household voltage is working. The part that makes the microwaves isn't.
In a conventional microwave, the heating circuit is a high-voltage transformer, a capacitor, a diode and the magnetron, with a fuse or thermal protector somewhere along the way. Any one of them can stop the heat. Inverter models, sold by Panasonic and others, replace the transformer with an inverter board, which becomes another suspect.
Testing those parts means working in the high-voltage section, so it is a job for a technician. What you can do is a simple test that confirms the symptom: put a mug of water in for sixty seconds on high, then feel it. If it is barely warm, the heating circuit has failed. If it is hot, the problem may be your container or the power level setting.
It sparks or arcs inside the cavity
Flashes of light and crackling inside the oven are worth stopping for right away. Common causes:
- •Metal in the oven. Foil, a twist tie, a fork, gold trim on a plate. Remove it and try again with a plain dish.
- •A burned waveguide cover. On the inside wall of the cavity there is a small flat panel, often a mica sheet, that covers the opening where the microwaves enter. Grease and food spatter soak into it and eventually char, and a charred spot arcs. You'll often see a black or brown mark on it. The cover is a replaceable part matched to the model.
- •Chipped interior paint. Where the coating inside the cavity has flaked away to bare metal, sparks can jump. Small chips can be touched up with paint made for microwave interiors; widespread rust inside the cavity is a reason to retire the unit.
- •Damaged rack supports on models that come with a metal rack. Use the rack only the way the manual describes.
Don't keep running a microwave that arcs. The heat can damage the magnetron and the cavity.
Nothing happens at all
A completely dark display has different causes than a microwave that won't heat.
Start outside the appliance. Plug a lamp into the same outlet. Check the breaker, and check whether the outlet is on a GFCI that has tripped somewhere else in the kitchen. Over-the-range microwaves are usually hard-wired to a dedicated outlet in the cabinet above, so the breaker is the first place to look.
If power reaches the outlet and the microwave is still dead, the likely culprits are a blown internal line fuse, a tripped thermal protector, or a failed control board. Thunderstorms are part of almost every summer afternoon here, and lightning can send a surge into household wiring that damages electronic controls. If the microwave went dark right after a storm, mention it when you book, along with anything else in the house that stopped working at the same time.
A blown line fuse isn't always the whole story. On many models, a door switch that fails or a door that is out of alignment is designed to blow that fuse on purpose. Replacing the fuse without finding out why it blew just leads to another one.
The door is part of the safety system
Microwave doors carry a set of interlock switches that cut power to the heating circuit the instant the latch opens. If the door is cracked, the latch is broken, the hinges are bent, or the door doesn't close flat against the frame, stop using the microwave until it is repaired. Never tape, wedge or bypass an interlock.
Symptoms of a switch problem include the unit starting by itself when the door closes, stopping mid-cycle when someone bumps the door, or refusing to start unless the door is pressed hard.
Turntable, light and fan
These are lower-stakes faults, and usually reasonable repairs:
- •Turntable not turning. Make sure the roller ring is seated and the glass plate is sitting on the coupler. If those are fine, the turntable motor under the floor may have failed.
- •Interior light out. On many models the bulb is reached from outside the cavity, sometimes through a panel on the vent grille. The manual shows where.
- •Vent fan on an over-the-range unit won't run, or runs on its own. Many over-the-range models switch the fan on automatically when the cooktop below gets hot. That is a protection feature, not a fault.
Care that actually helps
- •Clean spatter off the ceiling and the waveguide cover regularly, so grease never gets the chance to char.
- •On over-the-range models, wash the metal grease filters underneath, and replace the charcoal filter if the unit recirculates air back into the kitchen instead of venting outdoors.
- •Don't run the oven empty. With nothing to absorb the energy, the magnetron takes the abuse.
Is it worth repairing?
A basic countertop microwave usually isn't worth a service visit once the magnetron or control board fails. Replacing it is often the sensible call, and a technician will tell you so. Over-the-range, built-in and drawer microwaves are a different story. They are tied to the cabinet cutout and the vent, and swapping one involves more than the price of the appliance. For those, a repair often makes sense. Our guide to repairing or replacing an appliance covers how to weigh it.
A technician diagnoses the microwave at your home, explains what failed and quotes the repair, and you approve the price before any work starts. More on the models we service is on the microwave repair page.
Common questions
Can I replace the fuse myself?
The fuse sits inside the cabinet, in reach of the high-voltage capacitor. It is not a safe homeowner job, and a blown fuse usually signals another fault that needs diagnosing.
Is it safe to use a microwave with a cracked door?
No. Stop using it until the door is repaired or the unit is replaced.