A microwave that lights up, spins the turntable and hums, but leaves your coffee cold, has a fault in its heating circuit. It's also a fault where the safe homeowner checks are few and the dangers of opening the case are real. This guide explains what's going on inside, what you can check without opening the case, and how to decide between repair and replacement.
Why you shouldn't open a microwave
A microwave's heating circuit uses a high-voltage transformer, a high-voltage capacitor and a diode to power the magnetron, the part that produces the microwaves. That circuit operates at voltages far higher than a household outlet.
The capacitor can hold a dangerous charge even after the microwave is unplugged, sometimes for a long time. Touching it, or the wiring connected to it, can cause a severe or fatal shock. Discharging it safely requires the right tools and training.
So: don't remove the outer case of a countertop microwave, and don't remove the internal panels of an over-the-range microwave. The checks below don't require either.
First, confirm it isn't heating
A simple test tells you whether the microwave is heating at all.
- Put a microwave-safe cup of water in the center, about one cup.
- Run it on full power for one minute.
- Check the water. It should be noticeably hot. Lukewarm or cold water means the microwave isn't heating properly.
If the water is hot but food doesn't cook evenly, the problem is usually the turntable not rotating or the food being too dense or frozen, not a heating fault.
Things you can safely check
Power level and settings
Make sure the power level hasn't been set low, and that a defrost, warm or demo mode isn't selected. Some microwaves have a demo mode where the display and turntable work but the magnetron never turns on. The manual explains how to turn it off.
The door
The door has several switches that must all close before the microwave will heat. A door that doesn't close firmly, a bent latch, or a buildup of food on the latch can cause intermittent or no heating. Open and close the door firmly, clean around the latch hooks, and see if the problem changes. Don't try to adjust or bypass the switches.
The outlet and circuit
Countertop microwaves draw a lot of current. If the microwave shares a circuit with a toaster, kettle or coffee maker, performance can suffer, or the breaker can trip. Plug it into a different kitchen outlet if you can. Avoid extension cords.
The turntable
If the turntable doesn't turn, food heats unevenly. Check that the glass tray and roller ring are seated correctly and that nothing is stuck under them.
What usually fails
When a microwave runs but doesn't heat, the cause is almost always in the high-voltage circuit:
- •The magnetron, which produces the microwaves.
- •The high-voltage diode or capacitor.
- •The high-voltage transformer.
- •A door switch or the thermal protection, which can stop the magnetron from running.
- •The control board, which may not be switching the high-voltage circuit on.
All of these require opening the microwave and working near the capacitor. They're repairs for a technician.
Other symptoms worth knowing
- •Sparking or arcing inside: Stop using the microwave. Remove any metal, foil or dishes with metallic trim. If sparks come from the side wall, the waveguide cover (a small panel, often mica or plastic) may be dirty or damaged. Food buildup on it can burn and spark. Don't keep using the microwave if the sparking continues.
- •A burning smell or smoke: Stop, unplug it, and don't use it again until it's checked.
- •The microwave stops working and the display goes dark: A fuse may have blown, often because of a failing door switch. The fuse is inside the case, so this needs a technician.
- •Over-the-range microwave fan or light not working: These are separate from the heating circuit. They're still repairs that involve removing panels, so have them checked.
Repair or replace?
This depends heavily on the type of microwave.
Countertop microwaves are relatively inexpensive, and a magnetron or control board repair can approach the cost of a new unit. For a basic countertop model, replacement is often the sensible choice.
Over-the-range and built-in microwaves are different. They're more expensive, and replacing one means removing the old unit, possibly patching or adjusting the mounting and cabinet cutouts, and matching the rest of the kitchen. For these, repair is more often worth it, especially for a door switch, a fuse, a fan or a control problem. Our article on what fails first in builder-grade appliances covers over-the-range microwaves in newer homes.
Our repair or replace guide covers how to weigh the cost of a repair against the age of the unit.
When to call a technician
Call for service if:
- •The water test shows no heating and settings, door and outlet check out.
- •There's sparking that doesn't stop after removing metal and cleaning.
- •The microwave smells burnt, blows its fuse, or trips the breaker.
- •An over-the-range unit's fan, light or controls stop working.
See our microwave repair page for what we check, and our page for GE microwave repair, a common brand in over-the-range installations.
Frequently asked questions
Is it safe to use a microwave with a damaged door?
No. If the door is bent, the glass is cracked, the seals are damaged, or the door doesn't close properly, stop using the microwave and have it checked.
Does unplugging a microwave make it safe to open?
No. The high-voltage capacitor can hold a dangerous charge after the microwave is unplugged. Leave the case closed.
My microwave works but seems weaker than before. Is that a fault?
It can be an aging magnetron, but first check whether it's sharing a circuit with another appliance and whether the power level is set to full. The water test is a good way to compare over time.