Your microwave hums, the turntable spins, the light is on, and the food comes out exactly as cold as it went in. Or it's dead, or it throws sparks, or it shuts off halfway through. Some of those problems you can sort out yourself in a few minutes. Others involve parts that can kill you even with the microwave unplugged. Knowing which is which is the most important thing in this article.
A safety warning before anything else
Never remove the outer cabinet of a microwave. Inside is a high-voltage capacitor that can hold a lethal charge long after the unit is unplugged. There is no safe "quick look" inside the case for a homeowner. Everything in this guide stays outside the cabinet or inside the cooking cavity.
Also, don't run a microwave with a bent door, a broken hinge, a cracked door window or a damaged seal. The door is part of what keeps microwave energy inside. If it's damaged, stop using the unit.
Confirm it's really not heating
Put a cup of water in a microwave-safe mug and heat it on full power for one minute. It should come out clearly hot. If it's barely warm or unchanged, the microwave isn't heating.
If it heats but more slowly than it used to, check whether you've accidentally set a lower power level, and whether the microwave is on a circuit shared with other appliances.
Checks that are safe to do
Power. If the display is dark, check the breaker and any GFCI outlets on the circuit, including ones in other rooms. Over-the-range microwaves are usually hardwired or plugged into an outlet in the cabinet above, so check the breaker rather than looking for a plug.
Demo or child lock mode. Many microwaves have a demo mode for showrooms that runs the light, fan and turntable without heating. The manual explains how to switch it off. A child lock can make the keypad seem dead.
The door. If the microwave won't start at all, or starts and stops, open and close the door firmly. The door has latch hooks that press switches inside the cabinet. A loose latch or a door that doesn't close squarely can stop the microwave. Look at the latch hooks for cracks. Don't try to adjust or bypass the door switches; they're interlocks that prevent the magnetron from running with the door open.
The turntable. If it doesn't turn, check that the glass tray is seated on its roller ring and the coupling in the center. Clean the track underneath.
After a thunderstorm. A power surge during a summer storm can scramble an electronic control. Switch off the breaker for a minute, then turn it back on. If the display stays dead or garbled, the control may be damaged.
Sparks and arcing
Sparks inside the cavity are alarming but usually traceable:
- Metal. Twist ties, foil, metallic trim on plates and some takeout containers can arc.
- The waveguide cover. This is a small panel, often mica or similar material, on the inside wall or ceiling of the cavity. Food splatter on it can burn and arc. Clean it gently. If it's burnt through or cracked, it needs replacing, and the manual will tell you whether your model's cover is user-replaceable.
- Chipped interior paint. Exposed metal where the cavity coating has chipped can arc. Stop using the unit until it's evaluated.
If arcing continues with an empty clean cavity and no metal, stop using the microwave and have it checked.
What a "not heating" fault usually is
When the microwave runs, lights up, spins and doesn't heat, the problem is in the high-voltage section: the magnetron that produces the microwaves, the high-voltage diode, the capacitor, the transformer, a thermal cutout, or the door switches and their monitor circuit. A blown internal fuse can also leave a microwave completely dead.
All of these live inside the cabinet. Testing them means discharging the capacitor properly and working around high voltage. That's a technician's job and only a technician's job.
Repair or replace?
This is a real question with microwaves, and the honest answer depends on the type.
Countertop microwaves are often cheaper to replace than to repair, especially if the fault is the magnetron. Unless it's a high-end model, replacement usually makes more sense.
Over-the-range microwaves are a different story. They're also the range hood: the vent fan and the cooktop lights live in the same unit. They're mounted to the wall and the cabinet above, and they're heavy enough that removal is a two-person job. A replacement has to match the mounting plate and the cabinet opening, or the installation gets complicated. Repairing a newer over-the-range unit is often worthwhile.
Built-in, wall and drawer microwaves are installed with trim kits or matched to a wall oven or cabinetry. Replacing one can mean redoing the trim or finding an exact-size model, so repair is often the better choice.
When we diagnose a microwave, we'll tell you if the repair isn't worth it. A technician comes to the house, confirms what failed, explains it and quotes the repair before any work starts.
Over-the-range vent fan and light problems
If the microwave works but the vent fan or the cooktop light doesn't, check the grease filters underneath first. Clogged filters make the fan struggle. The filters usually slide out and can be soaked in hot soapy water. Charcoal filters for recirculating installs need replacing, not washing; the manual gives the part number.
Light bulbs under an over-the-range microwave are usually replaceable from below with power off. The manual shows how.
If the fan runs on its own and won't shut off, some models turn the fan on automatically when they sense heat from the cooktop below. That's protecting the electronics. If it runs with the cooktop cold, have it looked at.
When to call a technician
Call for a diagnosis if:
- The microwave runs but doesn't heat
- It's completely dead and the breaker and outlet are fine
- It trips the breaker when you start a cycle
- There's a burning smell, buzzing or loud humming
- Arcing continues with a clean, empty cavity
- The door is damaged or won't latch properly
See our microwave repair page for the types we work on, including Sharp microwave drawers and GE over-the-range models.
Frequently asked questions
Can I replace the magnetron myself?
No. Replacing it means opening the cabinet and working around the high-voltage capacitor, which can hold a deadly charge even when unplugged. This is not a do-it-yourself repair.
Is it dangerous to run my microwave empty?
It can damage the magnetron. With nothing to absorb the energy, it reflects back into the microwave. Don't run it empty, even briefly to test.
My microwave makes a loud humming noise but doesn't heat. What's wrong?
A loud hum or buzz with no heat often points to the high-voltage section, such as the magnetron, diode or capacitor. Stop using it and have it diagnosed.
Why does my over-the-range microwave fan turn on by itself?
Many models switch the fan on automatically when the cooktop below gets hot, to protect the electronics. It usually turns off once things cool down.