Microwaves troubleshooting: a microwave is a magnetron and a door switch.
A microwave either heats or it does not, and the parts in between are few. Most of what you can safely check is at the door, the keypad and the outlet. The high-voltage side under the cover holds a lethal charge even unplugged — that part is never a home repair.
First use: what is normal
A new machine can do things on its first run that look like a fault and are not. Here is what the makers say is normal, what to do first, and where the line is.
Normal the first time
- A faint electrical or hot-plastic smell for the first few uses.
- On an over-the-range model, a fan that starts by itself and keeps running. It is cooling the microwave and may stay on up to an hour while the range below is in use, and it cannot be turned off then.
Do this first
- 1Take out all packing, inside and out, before the first use.
Where the line is. A burning smell that is not food, or a smell that does not fade after several uses, is not normal: stop using it, unplug it and text us.
Before you touch anything
The install rules that decide most of what follows. Check these once; they rule out the causes that look like a broken machine and are not.
A microwave needs its own grounded 120V outlet, not a GFCI (nuisance trips) and not a shared circuit with the disposal.
An over-the-range unit needs 30 inches above a gas cooktop and the fan and damper set for the vent you have (out the top, out the back, or recirculating through a charcoal filter).
Never run it empty. Sixty seconds with nothing inside can damage the magnetron.
Runs, light on, turntable turns, but food stays cold
In this order
- Try a cup of water for one minute on high. Warm at all?
- Is the unit on a cooking cycle, or a timer or defrost setting?
- Any humming or buzzing louder than usual during the cycle?
What is usually behind it
The magnetron, the high-voltage diode or the capacitor has failed. The lights and the motor run on the low-voltage side, so everything looks normal.
What to do
This is a service call. On a countertop unit the part cost is close to the unit's price; on an over-the-range unit the factory warranty covers it.
The high-voltage capacitor holds a lethal charge for days after the unit is unplugged. The cover stays on; a tech opens it.
Sparks or arcing inside
In this order
- Metal in the cavity (a twist tie, foil, a gold-rim plate, the metal rack in the wrong position)?
- Is the waveguide cover (the small mica panel on the cavity wall) burnt, greasy or peeling?
- Is the cavity paint chipped to bare metal?
What is usually behind it
Metal in the chamber, a grease-soaked waveguide cover, or chipped cavity paint.
What to do
Remove the metal. Replace the waveguide cover (a cheap part, one screw). Touch up chipped paint with microwave cavity paint. Stop using the unit until the arcing source is gone — arcing burns through the wall.
Keypad dead or some keys will not respond
In this order
- Is the control lock on (hold the Start or Lock key three seconds)?
- Unplug for five minutes — does it come back?
- Is the keypad wet from a steam cleaning or a boil-over?
What is usually behind it
A control lock, a board that lost its place, or a membrane keypad that got wet or wore through.
What to do
Clear the lock, do the unplug reset, let a wet keypad dry a day. A keypad with the same dead keys after that is a membrane part.
Door will not latch, or the unit will not start with the door shut
In this order
- Are the latch hooks on the door intact and clicking into the frame?
- Does the unit start with firm pressure on the door?
- Did the door take a hit?
What is usually behind it
A broken latch hook or a door interlock switch — microwaves have three, and all must agree before it will run.
What to do
A latch hook is a door part; the interlock switches are behind the control panel and a tech's job, since the sequence has to be right for the unit to run safely.
Fan runs on its own (over-the-range)
In this order
- Is a burner on below it? Most OTR units start their fan automatically when the cooktop heats the housing.
- Did it start after a long cooking session?
- Is the vent fan setting stuck on?
What is usually behind it
The automatic exhaust — a thermostat in the housing turns the fan on when the cooktop below warms it, and off when it cools.
What to do
Nothing to fix; it stops when the housing cools. A fan that runs all day with the cooktop cold is a stuck thermostat.
Loud hum, buzz or rattle
In this order
- Is the turntable seated on its hub and the roller ring in place?
- Is the noise during heating only (the magnetron's normal hum is a steady, low tone)?
- OTR: are the grease filters seated so they do not rattle?
What is usually behind it
A turntable or roller ring off its seat, a loose filter, or (a harsh buzz with no heat) a failing magnetron or diode.
What to do
Seat the tray and the filters. A harsh buzz with cold food is the high-voltage side — a service call.
Completely dead
In this order
- Lamp in the same outlet?
- GFCI tripped?
- OTR: is the cord plugged into the outlet inside the cabinet above it?
What is usually behind it
The outlet or breaker, or an internal fuse that blew when a door switch failed.
What to do
Reset the GFCI and the breaker, check the plug in the upper cabinet. An internal fuse that blew once will blow again — the cause is a door switch, and a tech replaces both.
When it's a warranty call
Every unit leaves with the manufacturer's full factory warranty. These are the faults that warranty exists for — once the checks above have ruled out the install, the power and the vent, call the maker's line with the model and serial, and text us if you want a hand with the claim.
Runs but will not heat — the magnetron and the high-voltage parts.
A door interlock that will not let the unit run with the door shut.
A control board that is dead after a five-minute unplug on a good outlet.
All units are surplus inventory (open box, cancelled order, customer return, overstock, or minor scratch & dent). Every unit includes the manufacturer's full factory warranty. Please inspect thoroughly in person before purchasing — cosmetic imperfections or missing accessories are possible.
Text us the model number and what it's doing — we'll point you at the next step.
A fix, a part, or a warranty call — the plate's model and serial plus one line about the symptom is all it takes to tell which.
These pages are educational. Every appliance is a little different — follow the manufacturer's manual and local code, and leave gas, water heater and 240V work to a licensed installer.