A microwave that runs, spins and hums but doesn't heat is one of the most common appliance complaints. It's also one where safety matters more than usual. Before anything else in this guide: do not open the outer cabinet of a microwave. That's not caution for its own sake. The reasons are explained below.
The safety warning that matters most
Microwaves contain a high-voltage capacitor. It can hold a dangerous, potentially lethal electrical charge even after the unit is unplugged, sometimes for a long time. Discharging it safely requires the right tools and training.
That means:
- •Never remove the outer cover to "take a look," even with the microwave unplugged.
- •Don't operate a microwave with a damaged door, broken hinges, a warped door or a cracked door seal. The door and its interlock switches keep microwave energy inside.
- •Don't bypass or tape door switches, even if the microwave seems to work better that way.
- •If you smell burning or see smoke, stop the microwave, keep the door closed and unplug it or switch off the breaker.
Everything we suggest below can be done without opening the cabinet.
Checks you can safely do
Does it have power?
- •Check the outlet with another small appliance.
- •Check the breaker. Over-the-range microwaves are often on their own circuit.
- •If the display is dark and nothing responds, an internal fuse may have blown. That repair requires opening the cabinet, so it's a job for a technician.
Is it heating at all?
Put a cup of water in for one minute on full power. If it's barely warm, the microwave isn't heating properly. If it's warm but not hot, the power level setting may be low, or output could be weak.
Is the door closing properly?
A microwave won't run, or may behave oddly, if the door switches don't register a closed door. Check that the latch hooks aren't broken and that nothing is caught in the door. Don't force a door that doesn't close cleanly.
Is the turntable turning?
If the turntable doesn't spin, check that the plate and roller ring are seated properly. A failed turntable motor is a common and usually modest repair.
Sparking inside the cavity
Sparks (arcing) can come from metal in the cavity, like foil, twist ties or dishes with metallic trim. If there's no metal and you still see sparks, check the small cover on the inside wall, usually made of mica or a similar material. If it's greasy, burned or cracked, stop using the microwave. Food splatter on the cavity walls can also arc. Clean the interior thoroughly with the unit unplugged.
What usually fails
When a microwave runs but doesn't heat, the cause is typically one of a small group of high-voltage parts:
- •The magnetron, which produces the microwave energy
- •The high-voltage diode
- •The high-voltage capacitor
- •The high-voltage transformer (less common)
Door interlock switches also fail, and a failing switch can blow the internal fuse. All of these sit behind the cabinet cover and involve high voltage. They're technician repairs.
Other common faults include a touchpad that doesn't respond, a failed control board, a stuck turntable motor and, on over-the-range units, a failed vent fan or light.
Countertop units: Often replace
For most countertop microwaves, the cost of diagnosis plus a high-voltage part often approaches the cost of a new unit. We'll tell you honestly if we think replacement makes more sense. If you do replace, make sure the old microwave is disposed of properly. Many cities in the area offer bulky-item or electronic waste collection.
Over-the-range and built-in units: Often worth repairing
Over-the-range microwaves double as range hoods, and built-in microwaves or microwave drawers are fitted into cabinetry. Replacing them isn't just buying a new unit. It can mean matching the mounting plate, cutout size and trim kit, and possibly adjusting cabinetry. That makes repair more attractive, especially on higher-end models.
Over-the-range and built-in units come from many brands, including GE, Whirlpool, KitchenAid, Sharp, Bosch and Viking, and parts are generally available for current and recent models.
Over-the-range units need extra care
- •Clean or replace the grease filters under the unit regularly. Clogged filters make the vent fan work harder.
- •Replace charcoal filters on recirculating models on the schedule in the manual.
- •If the vent fan runs by itself, many models do this automatically when they sense heat from the cooktop below. It's a safety feature, not necessarily a fault.
Power surges
Brief outages and voltage dips during summer heat waves can damage control boards. If your microwave's display went blank after a power event, try switching off the breaker for a minute and restoring power. If it doesn't come back, the control or an internal fuse may need attention.
When to book a technician
Book a repair if:
- •It runs but doesn't heat
- •The display is dead and the outlet and breaker are fine
- •You see sparking with no metal inside and a clean cavity
- •The door doesn't close properly or the latch is broken
- •The vent fan or light on an over-the-range unit has failed
How we diagnose microwaves
We safely discharge the high-voltage capacitor before testing anything, then check the fuse, door switches and high-voltage components with the proper instruments. After diagnosis we'll tell you whether the repair makes sense for your unit's type and age. More details are on our microwave repair page. If you're on the fence, our guide to repairing or replacing an appliance may help.
Frequently asked questions
Is it safe to use a microwave that sparks sometimes?
No. Stop using it until you've found the cause. Metal, food residue or a damaged cover inside the cavity are common reasons.
Can I replace the magnetron myself?
We strongly advise against it. The job requires opening the cabinet and working near a capacitor that can hold a dangerous charge.
Why does my over-the-range microwave fan turn on by itself?
Many models run the fan automatically when they detect heat from the cooktop below. It's normal behavior.