A microwave that lights up, turns the plate and counts down but leaves the food cold has almost always lost one of four parts: the magnetron, the high-voltage diode, the high-voltage capacitor, or one of the door switches. Occasionally it is a blown fuse in the high-voltage circuit.
All of those are inside the cabinet. Before going any further, there is something about that cabinet you need to know.
Read this before you take the cover off
The high-voltage capacitor inside a microwave stores enough charge to kill you, and it holds that charge after the appliance is unplugged. Not for seconds, but potentially for a long time. Unplugging a microwave does not make it safe to work on. The capacitor has to be deliberately discharged first, with the right tool and the right technique.
This is different from every other appliance in your kitchen. You can safely open a dishwasher or a dryer once it is unplugged. A microwave is the exception, and it is the reason this article stops where it does.
Everything below explains what has failed and why, so you know what you are being told when a technician diagnoses it. It is not a repair guide, and deliberately so.
Why is my microwave not heating?
Work through these in order of how often they turn out to be the cause.
The magnetron
The magnetron is the component that actually produces the microwaves. Everything else (the turntable, the light, the fan, the timer) runs off the ordinary low-voltage side of the appliance, which is exactly why a dead magnetron leaves you with a microwave that seems perfectly alive and cooks nothing.
Magnetrons wear out. Most last somewhere between seven and ten years of regular use, less if the appliance has been run empty or with very small loads, which reflects energy back into it.
The clearest sign is a microwave that has gone from heating slowly over a few weeks to not heating at all. Magnetrons often weaken before they fail outright. A loud buzzing or humming that started around the same time points the same way.
The magnetron is also the most expensive part in the machine, and on most domestic microwaves the cost of the part plus the labor lands close to the price of a new unit. For a built-in or over-the-range model the sums are different, because replacing the appliance means matching the cabinet opening.
The high-voltage diode
The diode works with the capacitor to double the voltage the magnetron needs. When it fails, the magnetron does not get enough voltage to produce anything, so the symptom is identical to a dead magnetron: running, cold food.
The difference is cost. A diode is a cheap part. This is why a proper diagnosis matters rather than assuming the worst: a fair number of microwaves written off as having a failed magnetron have a failed diode instead.
A failing diode often makes a loud buzzing under load, and sometimes trips the breaker when it shorts.
The door switches
Microwaves have three or four interlock switches around the door. Their job is to make absolutely certain the appliance cannot produce microwaves with the door open. They are wired so that if one fails, the whole heating circuit shuts down.
A failed interlock is a common cause of a microwave that runs but does not heat, and it is one of the cheaper repairs. It also tends to announce itself: the door needs a firmer push than it used to, or the latch has started catching. If the door has been slammed for years, or the latch is visibly worn, the switches are worth checking early.
Microwave door switch replacement is a small job in parts terms. It is still inside the cabinet, so the capacitor warning above still applies.
The fuse
Microwaves have more than one fuse. The main line fuse protects the whole appliance, and if that goes the microwave is completely dead, with no light, no display, nothing. That is not the fault this article is about.
What matters here is that a blown microwave fuse in the high-voltage circuit can kill the heating while leaving the rest working. And more usefully: a fuse that has blown has usually been blown by something else. Fitting a new one without finding what caused it means the new one goes too.
The two things you can check without opening anything
Both take a minute and both rule things in or out.
First, test it properly. Put a cup of cold water in and run it for a minute. If the water is warm, the microwave is heating and the problem is the food or the settings, not the appliance. It sounds obvious, but power-level settings get changed by accident more often than you would think. A microwave stuck on power level 3 heats so slowly it looks broken.
Second, look at the door. Push it closed and feel whether it seats the way it used to. A door that has dropped on its hinges, or a latch that no longer clicks cleanly, stops the interlocks closing properly. That is a real fault with a visible symptom and it is the cheapest one on this list.
Repair or replace
For a countertop microwave, the honest answer is usually replace. A magnetron replacement on a mid-range countertop unit costs close to what a new one costs, and a technician who tells you otherwise is not doing you a favour.
For a built-in or over-the-range microwave, repair almost always wins. Those units are expensive, and replacing one means matching the cabinet cut-out and the venting arrangement, which turns a swap into a small kitchen job.
The part that changes the maths is the diagnosis. A door switch or a diode is an inexpensive repair on any microwave, including a cheap one. It is only worth writing the appliance off once you know it is the magnetron, which is the whole argument for having it looked at rather than guessing.
Common questions
Why is my microwave running but not heating?
The turntable, light and timer run on the low-voltage side, and the heating runs on the high-voltage side. A fault on the high-voltage side (magnetron, diode, capacitor or a door interlock switch) stops the heating while everything else carries on as normal.
Is it safe to open a microwave to fix it?
No, not without training. The high-voltage capacitor stores a charge capable of killing you and holds it after the appliance is unplugged. It has to be discharged deliberately before anything inside is touched. This is the one kitchen appliance where unplugging it is not enough.
How long does a microwave magnetron last?
Usually seven to ten years of regular use. Running the microwave empty or with very small quantities shortens that, because the energy reflects back into the magnetron instead of being absorbed by food.
How much does it cost to fix a microwave that is not heating?
It depends entirely on which part failed. A door switch or a diode is an inexpensive repair. A magnetron is the most expensive part in the appliance, and on a countertop model that repair often costs close to a replacement. The diagnosis is what determines the answer.
Can a blown fuse stop a microwave heating?
Yes. A microwave has more than one fuse, and one in the high-voltage circuit can stop the heating while the display and turntable keep working. A blown fuse is usually a symptom of another failure, so replacing it alone rarely holds.
Getting it looked at
If the cup-of-water test comes back cold and the door is seating properly, the fault is inside the cabinet and it needs someone with the equipment to discharge the capacitor safely. AGA Appliance Service covers 29 cities across seven counties around Charlotte and into northern South Carolina, 9 AM to 8 PM every day, with same-day availability on bookings before 11 AM. Diagnosis is $79 and waived when you go ahead with the repair, so you find out whether it is a $20 diode or a magnetron before deciding anything. Repairs carry a 90-day parts and labor warranty.