A microwave that lights up, turns the plate and counts down but leaves the food cold is a specific fault with a short list of causes. It is also the appliance where the repair-or-replace answer depends most on what kind of microwave it is.
First: a genuine safety point
A microwave's high-voltage capacitor stores a charge that can be lethal, and it holds it after the appliance is unplugged. This is not the usual boilerplate caution — it is the reason microwaves are the one appliance we tell people not to open.
Everything below is for understanding what is happening, not for doing it yourself.
What actually causes "runs but does not heat"
- •The door interlock switches. The most common failure by a wide margin, and they are also the safety system — the appliance will not produce microwave energy unless all of them confirm the door is closed. Nothing else is assessed until they are checked.
- •The magnetron, which is the component that generates the energy. When it fails, everything else works perfectly and the food stays cold.
- •The high-voltage diode or capacitor, which feed the magnetron. A failed diode is a common cause and presents identically to a failed magnetron.
- •The high-voltage transformer, less common and more expensive.
From outside the cabinet these are indistinguishable. That is why the diagnosis has to happen on site and why nobody honest quotes it from a description.
The other complaints
Sparking or arcing inside. Stop using it. Usually the waveguide cover — a small mica panel on the side wall — has burned through, or there is food debris carbonised on it. It is an inexpensive part, but running the appliance in that state damages more than the cover.
Turntable not turning, food heating fine. The turntable motor or the coupler. Cheap and simple.
Buttons not responding. The membrane keypad or the control board. On an over-range unit this is worth fixing; on a countertop unit it usually is not.
Fan running loudly on an over-range unit. That is the ventilation blower, which on these doubles as the kitchen's extraction. The filters above the cooktop are part of the service and are frequently the whole problem — grease-loaded mesh filters make the blower work far harder than it should.
Which microwave do you have?
This decides the answer.
Countertop. A replacement costs little enough that a magnetron, diode or board repair rarely makes financial sense. We will tell you that on the phone if you describe the symptom, rather than sending someone out to confirm it for a fee.
Over-the-range. Different conversation. These are fitted to the cabinetry, ducted into the kitchen's ventilation, and cost substantially more to buy and install. A door switch, a magnetron, a diode, a blower or a keypad is worth repairing here.
The Chicago detail worth knowing: in a lot of apartment and condo kitchens the over-range microwave is the *only* ventilation in the room, and it is often recirculating rather than ducted outside because running a duct through a hundred-year-old masonry wall was not an option during the rehab. That makes the charcoal filter as well as the mesh filters part of the maintenance, and a failed blower a bigger problem than it looks.
Built-in and drawer microwaves. Fitted into a cabinet run, frequently part of a matched suite. Replacement means matching an opening and often a cabinetmaker, so these stay worth repairing a long way past the point a countertop unit would be replaced.
Straight answer
If it is a countertop microwave that will not heat, replace it. We will say so rather than take the job.
If it is over-range, built-in or a drawer unit, call for a diagnosis. The failure list above spans a wide enough price range that it is worth knowing which one you have, and on that equipment most of them are worth fixing.