I have a very nice over-the-range microwave that died after 6 years of use (or after 8 months of renting to a very messy person). To replace with the same model used would be cost over $250, thus I decided to trouble shoot it.

If i place a cup of water in the microwave everything seems to work, but the water does not get hot.

I verified the mode stirrer rotates freely, and that it's motor actively turns when the microwave is on. The inside of the wave-guide, the motherboard, and the magnetron are surprisingly clean given the state the rest of device was in.

Because the magnetron is the most expensive part in the microwave, I'm a bit hesitant to order one and try swapping it out.

Are their any other parts besides the mode stirrer I should consider testing / replacing before replacing the magnetron?

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3 Answers 3


Somewhere on the magnetron is a thermal fuse or cutoff. When the magnetron gets to hot, it shuts off power to the device. Some of these fuses reset once the device cools, some don't. Sounds like you have one of the later. You can replace the fuse, temp rating is usually written on the device itself. While you're in there, clean the cooling fins on your magnetron with 409 or similar cleaner and a toothbrush. Buildup of kitchen grease on them can make the magnetron run too hot. Also check that the cooling fan, if any, is working, has a free air path, and is pointed at the magnetron fins. When my microwave started cutting out, all it took was a good magnetron cleaning to get it running again. Sounds like your thermal fuse is a one-shot though. You might replace it with a auto-reset version.

  • I swapped with another magnetron, but had no luck; I'll hunt for the temperature fuse tomorrow and report back if it's separate.
    – virtualxtc
    Apr 27, 2014 at 7:24
  • I'm having trouble finding the temperature fuse.
    – virtualxtc
    Nov 2, 2014 at 22:28
  • They can look like this: google.com/… Nov 2, 2014 at 23:51
  • But on microwaves I usually find something like this: google.com/… It'll be screwed down somewhere near the magnetron. Nov 2, 2014 at 23:52

Search for sci.electronics.repair and look up their microwave troubleshooting chart. Check to see if the primary side of the high voltage transformer is getting 120V to rule out any switch/relay/thermal switches/other circuitry not giving it power. If you are getting power now start troubleshooting the high voltage side. Write down and mark where each high voltage wire goes and take a picture for reference. Disconnect the transformer and check to see if it is open or shorted on all windings. If that's not a problem, disconnect the diode, put it between a 9volt battery and your tongue to see if there is voltage going through. It should only conduct one way. You can also use a voltmeter and a 9volt battery to measure the voltage, will be much less. You cant use continuity testing on these diodes as their forward voltage is too high.

battery + ---|>|---o voltmeter measure +

battery - ---------o voltmeter measure -

Check the capacitor, it should measure 10 or 20 Mohm as there is an internal resistor. Your measurement may start off at a high point and then climb down. That is normal. You can look up videos online on how to test the capacitor. If you have a capacitance setting on your meter, verify if it is within range specified on it. Finally make/buy a resistor divider to divide the voltage from 5000 or so volts down to something low that your meter can handle. Disconnect the magnetron and connect your divider with meter to the high voltage wires that used to connect to it. If you are getting high voltage and still no heat, its most likely the magnetron.


Sounds like the magnetron is faulty. Remove it and use a multimeter to test between one pin and the body and then also test from the second pin to the body of the magnetron. If there is resistance , change it. They are cheap on eBay . The fuse from the large capacitor to the large transformer is usually blown as well. Good luck

  • if you check my comment on Wayfaring's answer back from April, you'll see I swapped out the magnetron and it did not fix the problem.
    – virtualxtc
    Nov 2, 2014 at 22:27
  • 2
    @virtualxtc, new information should be added to the question, and if the nature of the question changes the title should be updated accordingly. It's not reasonable to expect your volunteer assistants to scour all comments on all questions in order to provide an answer.
    – isherwood
    Sep 25, 2017 at 15:17

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