Start with the part of this article that matters most: do not open the cabinet of a microwave. Inside is a high-voltage capacitor that can hold a dangerous charge after the oven is unplugged, sometimes for a long time. It's one of the few household appliances where a curious look inside can seriously hurt you. Everything suggested below can be done from the outside, with the door open or the unit unplugged, and nothing requires removing the outer cover.
With that said, a lot can be learned from what the microwave is doing.
Which microwave do you have?
The type decides whether a repair is worth considering at all.
- •Countertop. Freestanding and plugged into an outlet on the counter. These are usually inexpensive to replace, so most repairs don't make financial sense unless the fix is small.
- •Over-the-range (OTR). Mounted to the wall cabinet above the cooktop, doubling as the range hood, with its own exhaust fan and cooktop lamp. Removing and remounting one is real work, and they often match the other kitchen appliances.
- •Built-in or drawer. Installed in a cabinet or under the counter, sometimes with a trim kit. Drawer models like Sharp's, and built-ins from brands such as Thermador, Wolf and Miele, fit specific openings. Repair usually wins when it's possible.
It runs, the light's on, the plate turns, but nothing heats
This is the classic microwave failure. The fan, light and turntable all run on ordinary household voltage. The heating comes from a separate high-voltage circuit: a transformer, capacitor, diode and magnetron. When any of those fails, you get a microwave that sounds normal and does nothing useful.
A simple check: put a microwave-safe cup of cold water in and run it on high for a minute. If the water is barely warmer, the high-voltage side isn't working. If it's warm but less than you'd expect, the oven may be weak rather than dead, or the power level may be set low by mistake.
Other possible causes, in rough order:
- •A door switch that isn't closing fully. A microwave uses a set of interlock switches, and every one of them has to register the door as closed.
- •A failed high-voltage diode or capacitor.
- •A failed magnetron.
- •A thermal cutout that has opened because the oven overheated.
- •On some models, the control board's relay for the high-voltage circuit.
All of these are inside the cabinet. This is a repair for a technician, full stop.
Completely dead: no display, nothing
First, the outside:
- Is the outlet live? Plug in a lamp or phone charger to test it.
- For an over-the-range model, check the breaker. OTR microwaves often have their own circuit.
- Check whether a GFCI outlet elsewhere on the kitchen counter has tripped, since it may protect other outlets.
If power is reaching it and it's still dead, the most common internal cause is a blown main fuse. Fuses in microwaves often blow because a door switch failed, so replacing the fuse alone can lead to it blowing again. That's why this should be diagnosed rather than guessed.
Sparks and flashes inside
Arcing is alarming, but it's often traceable.
- •Metal in the oven. Foil, twist ties, dishes with metallic trim or gold edges, even some takeout containers with metal handles.
- •The waveguide cover. On the inside wall, usually on the right side or ceiling, there's a small panel of mica or plastic-like material. Grease soaks into it over time, and it can burn and spark. It's replaceable.
- •Chipped interior paint. Exposed metal inside the cavity can arc. Small chips can be repaired with the correct microwave cavity paint; widespread rust and peeling usually means replacement.
- •A metal rack left in during microwave-only cooking, on convection models whose manual says to remove it.
If arcing continues with an empty, clean oven, stop using it until it's checked.
Turntable, keypad, and door problems
- •Turntable won't turn: lift the glass tray, reseat it on the roller guide, and look at the three-lobed coupler in the center for cracks. If everything sits properly and it still won't rotate, the small motor under the floor is a routine replacement.
- •Some keypad buttons stopped working: membrane keypads wear out, and steam or spills can get under them. Depending on the model, the keypad or the control board may be replaceable.
- •The door won't latch, or the microwave starts and stops when you touch the door: the latch hooks or door switches are worn. Don't use a microwave whose door doesn't close properly.
Over-the-range models: the vent side
An OTR microwave is also your range hood. Problems there include:
- •The vent fan runs by itself. Lots of OTR units have a sensor that starts the fan by itself when the burners underneath get hot. That's a protection feature, not a fault. It should stop once things cool down.
- •The fan is noisy or dead. The fan motor is a replaceable part.
- •Weak ventilation. Wash the metal mesh filters on the underside in hot soapy water. Units set up to recirculate (not ducted outdoors) also have a charcoal filter behind the top grille that wears out and should be swapped periodically.
- •The cooktop light is out. Usually a bulb, which the manual shows how to change.
Because an OTR is exposed to heat and grease from the cooktop, keep its filters clean and use the fan when cooking.
Is it worth repairing?
Being direct about this saves everyone time.
A small countertop microwave with a failed magnetron is often not worth repairing, and we'll tell you that. An over-the-range microwave, a built-in speed oven, or a microwave drawer is a different story. Replacing one means matching the opening, the mounting and sometimes the trim kit, and those models cost more to replace. For them, a repair frequently makes sense.
Some other factors that tip the decision are covered in our repair or replace guide and our article on what goes into a repair quote.
Getting it fixed
Book through our microwave repair page. The technician discharges the capacitor properly, tests the high-voltage parts, door switches and controls, and gives you the diagnosis and a price before touching anything else. We repair common brands, including GE, Whirlpool, Samsung and Sharp, with parts matched to the model number. If a wall oven or range is also giving you trouble, see our oven repair guide and mention both when you book.