If the microwave lights up, the fan runs, and the turntable moves but food stays cold, rule out settings and simple operating conditions before assuming the magnetron has failed. The magnetron generates the microwave energy used for heating, but it is not a do-it-yourself repair component.
Run a simple water test
Check your model manual. Many manufacturers suggest heating a cup of water in a microwave-safe container for about a minute. If the water barely changes temperature, you have confirmed a heating problem rather than a food-specific issue.
Check Demo mode and power level
Some microwaves have a retail Demo mode in which the light, fan, and turntable operate without energizing the magnetron. Also make sure the power level is not set very low and that you did not choose defrost or delayed start by mistake.
Inspect the door without disassembling anything
The door must close completely. Debris around the latch or a damaged door mechanism can interfere with normal operation. The FDA says not to operate a microwave with damaged hinges, latches, seals, or a door that does not close firmly.
If it heats nothing, internal repair belongs to a technician
Whirlpool lists a faulty door switch, high-voltage diode, magnetron, and other internal components among possible causes. Diagnosing them requires disassembly around high-voltage circuits. Do not remove the outer cover even when the microwave is unplugged.
Separate “not heating” from “heating unevenly”
If the water test works but food has cold areas, look at portion size, time, power, turntable movement, and cookware. Microwave heating is not perfectly uniform in every food.
Stop using it when there are warning signs
Unplug and arrange service for burning odor, repeated arcing, a new loud electrical noise, a damaged door, or any operation with the door open. On an older unit, compare repair cost with replacement.
Home troubleshooting ends with settings, power supply, and the door. High-voltage diagnosis is where professional service begins.
Sources
- Whirlpool — Microwave Not Heating: Possible Causes and Fixes — Demo mode, power level, door switch, magnetron, and other causes.
- FDA — Microwave Ovens — Door safety and manufacturer-instruction guidance.