---
title: "A Microwave That Runs and Does Not Heat, and the Honest Arithmetic"
description: "LG microwave problems when it runs but does not heat: the one-cup test, what actually fails, and when repairing a countertop machine stops paying for itself."
url: "https://lgmiami.support/blog/microwave-runs-does-not-heat/"
date_modified: "2026-09-05"
Category: "Common Problems"
---


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

## 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.

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 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.

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