LGRepair Miami
(305) 419-0291 Book a visit

Common Problems

A Microwave That Runs and Does Not Heat, and the Honest Arithmetic

The one-cup test settles the diagnosis. Put a plain glass of cold water in the middle of the cavity, run the machine at full power for one minute, and feel it. Water that is noticeably warm means the machine is producing power and the complaint is about time or quantity. Water exactly as cold as it went in confirms the magnetron circuit — the magnetron itself, the high-voltage diode, the capacitor or the transformer — and nothing else in the machine has to be wrong for that to happen, which is why everything visible keeps working. As for cost, the honest answer depends on where the machine lives rather than on which part failed. On a countertop machine the parts and the labor together often approach the price of a new one and we will say so. On a built-in or an over-the-range unit, repairing is nearly always right, because most of what you paid for is the cabinet, the trim and the ventilation rather than the microwave.

Book a diagnostic (305) 419-0291

The full guide

Common Problems By Appliance Content Team Last checked 3 minute read

LG microwave problems: the one-minute test that settles it

Put a plain glass or mug of cold water — about a cup — in the middle of the cavity. Run the machine on full power for one minute. Then feel the water.

Noticeably warm means the machine is producing microwave power. Whatever the complaint is, it is about time, quantity or expectation rather than a failure.

Exactly as cold as it went in confirms it. That is a specific diagnosis, not a vague one, and it points at one circuit.

Use a plain glass without markings, and do not use an empty cavity — running a microwave with nothing in it is hard on the magnetron and proves nothing.

Why everything else still works

A microwave is two nearly independent machines in one box.

The first is the part you can see: turntable, light, cooling fan, timer, control panel. All of it runs on ordinary household mains.

The second is a high-voltage circuit that produces several thousand volts through a transformer, a capacitor and a diode, and feeds a magnetron — the component that actually makes the microwaves.

Two circuits in one cabinet, and where the drawing stops A microwave is two machines sharing a cabinet. The first runs on ordinary household voltage: the turntable motor, the interior lamp, the cooling fan, the touch panel and the clock. None of it has anything to do with cooking, which is why a machine that turns, lights, counts down and beeps can leave the food exactly as cold as it went in. The second is the high voltage circuit, where a transformer steps household voltage up enormously and a capacitor, a diode and a magnetron produce the energy that heats food. The two share the control board and nothing else. The capacitor stores a charge that can injure or kill, and it holds that charge long after the appliance has been unplugged, so the boundary between the two circuits is where this drawing ends rather than continuing into a list of checks. TWO MACHINES IN ONE CABINET. THEY SHARE THE BOARD AND NOTHING ELSE. ORDINARY HOUSEHOLD POWER The turntable motor The interior lamp The cooling fan, the panel, the clock None of it has anything to do with cooking. A SEPARATE HIGH-VOLTAGE CIRCUIT A transformer, stepping up enormously A capacitor and a diode A magnetron, which makes the heat This is the half that has stopped. It turns, lights, counts down and beeps — and the mug comes out cold. That is what this looks like. THE CAPACITOR HOLDS ITS CHARGE AFTER THE PLUG IS OUT Which is why this drawing ends here instead of carrying on into a list of things to check.
Unplugging it does not make the inside safe, and neither does waiting overnight. Stop at the cabinet: everything you can observe from the kitchen is a report about the half that was never the problem.

So a machine where everything visible works perfectly and nothing gets hot is not a mystery. It is the second half, and only the second half. The turntable turning is evidence rather than a contradiction.

What actually failed

Four parts produce this symptom and they cost very different amounts:

  • The magnetron — the expensive answer, and the one people assume.
  • The high-voltage diode — much cheaper, fails often, identical symptom.
  • The capacitor — likewise.
  • The transformer — less common, and it usually announces itself with a smell.

That spread is exactly why a phone quote here is guessing. The symptom does not name the part.

The arithmetic, said plainly

This is the part of the page that matters more than the diagnosis.

A countertop machine. The parts and the labor together often approach the price of a new machine of similar output. We will tell you that rather than sell you a repair that does not make sense. There is no version of this trade where talking somebody into a bad decision is good business.

An over-the-range machine. Nearly always worth repairing. You are not just buying a microwave — you are buying the cabinet fit, the trim, the ventilation and the fact that it works with the cooktop underneath it. Replacing one means matching a size and a finish and re-doing the install.

The microwave half of a combination wall oven. Definitely worth repairing. It is half of a built-in appliance and it cannot be swapped for a countertop machine at any price.

The repair-or-replace line on a mid-market machine, pulled two ways Two forces act on the decision at once. Moving the line earlier: the machine was inexpensive new, it is freestanding and slides out on its own, and there are no panels or joinery to match. Moving the line later: the parts belong to major manufacturers such as Whirlpool, LG and Frigidaire, those catalogs are still stocked, and the builder wrote real service documentation. Only two repairs approach the price of the machine itself — a sealed system, meaning compressor, evaporator and charge, and a main control board on an older unit. Everywhere else the arithmetic still favors the repair. No figures appear here on purpose: each one would be a range until somebody has seen the machine. A FREESTANDING MACHINE IS NOT THE BUILT-IN ARGUMENT. MOVES THE LINE EARLIER It was inexpensive new Freestanding — it slides out No panels, no joinery to match MOVES THE LINE LATER Whirlpool, LG, Frigidaire parts Real catalogs, still stocked Documentation the builder wrote TWO REPAIRS APPROACH THE PRICE OF THE MACHINE A sealed system — compressor, evaporator, charge A main control board on an older machine Everywhere else the arithmetic still favors the repair. No figures here on purpose: every one of them is a range until somebody has seen the machine, and a range in a box reads as a quote. The price is agreed after an on-site diagnostic.
Two of these repairs are a real decision. The rest, on a machine with years left in it, are not close.

The one thing that is never a wait-and-see

If the machine starts by itself when you close the door, unplug it now.

That is a failed door interlock: the switch that proves the door is shut has stuck closed, and the safety chain that prevents microwaves being produced into a room has lost part of itself. It is a small, inexpensive part and it is a safety part, and a microwave that can run on its own does not stay on the counter while you think about it.

And the thing not to do

Do not open the case, and this is the one appliance where we say that without qualification. The capacitor inside stores a charge that can be lethal, and it holds it long after the machine has been unplugged — weeks, in some cases. Every internal job on a microwave starts by discharging it deliberately, with the right tool, and that is not a step to improvise.

If it turns out to be a repair

Every job on this site is priced the same way: an on-site diagnostic first, the figure agreed in writing before anybody starts.

  • A mug of water standing on the glass turntable of a countertop microwave with the door open and the cavity light on, the water plainly still and cold, steam nowhere.
    It runs, the light is on, and nothing gets hot

    Microwave not heating

    One of the most specific symptoms in this trade. Everything else about the machine works — the turntable turns, the fan runs, the light comes on and the timer counts down — and the food stays cold. That combination points at one circuit, and it is worth an honest conversation about what repairing it costs.

    • The magnetron circuit: magnetron, diode, capacitor or transformer
    • Nothing else in the machine has to be wrong for this to happen
    • On a built-in or over-the-range unit repair is usually right; on a countertop it may not be
    What it usually means
  • An insulated screwdriver laid across the terminals of a high-voltage capacitor inside an opened countertop microwave cabinet, the cover off beside it on the bench.
    from $185

    Microwave repair

    Built-in and over-the-range machines, where the value is in the installation as much as the appliance. The magnetron circuit, the door interlocks and the control cover nearly everything that goes wrong. On a countertop machine we will tell you plainly when the arithmetic does not favor repairing it.

    • A machine that runs and does not heat is the magnetron circuit, specifically
    • A door interlock that has failed is a safety repair and comes out of service today
    • The capacitor holds a lethal charge — this is the one appliance not to open
    What this involves

Questions people ask about this

Is it dangerous to keep using it?

A machine that runs and does not heat is producing nothing, so there is nothing leaking. The dangerous case is the reverse: a door that does not seal, or an interlock that has failed so the machine can start on its own. That one comes out of service the same day. What is genuinely dangerous in every case is opening the case — the capacitor stores a charge that can be lethal weeks after it was unplugged.

It heats, but slowly.

That is usually not the magnetron, which tends to work or not work rather than fade. Slow heating is more often low supply voltage, a shared circuit under load, or simply a machine of lower wattage than the one it replaced. Worth measuring before anything is replaced.

How much does an appliance repair cost in general?

It divides more by access than by appliance. A part that is reachable from the front of a freestanding machine is one kind of job; the same part behind joinery, or above a cooktop at head height, is another. Every figure on this site is a from, and the exact number is confirmed on site after the diagnostic and agreed in writing before anything is opened.

Can the turntable motor cause this?

No, and that is a useful thing to know. The turntable, the light, the fan and the timer all run on ordinary mains and have nothing to do with the high-voltage circuit that actually cooks. Their working perfectly is the diagnosis rather than a contradiction of it.

Common Problems

  • A shallow black drain pan slid out from underneath the base of a refrigerator that stands pulled forward on a tile floor, a little dirty water still lying in it.

    Water Under the Crisper Drawer, and the Tube Behind It

    A refrigerator handles two entirely separate kinds of water, and where the puddle is decides which one you have before anybody is called.

    Read more
  • A short drain tube pulled out beside a washing machine's pump filter with its cap off, running into a shallow tray on the floor, towels laid around it.

    A Washer Stopped Full of Water: The Filter Nobody Opens

    There is a filter behind a small door at the bottom of your washer, it is designed to be opened by you, and what comes out of it is usually the whole explanation.

    Read more
  • A glass jug held under a refrigerator's water dispenser and filling steadily, the dispenser recess lit, the rest of the kitchen dark.

    No Ice, But the Water Dispenser Works: What That Narrows It To

    One ten-second test splits this problem in half, because ice and water share a supply and part company inside the door.

    Read more

Tell us the model number and what it is doing.

Schedule a visit Call (305) 419-0291

This article is based on:

  • Official manufacturer documentation
  • Internal repair procedures
  • Technician repair history
  • Company quality standards
Call (305) 419-0291