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Magnetron stays on after timer ends. Opening the door turns it off, but it turns back on when door closed. Cancel/off button has no effect. Only way to get out of this failure state is to unplug it.
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Can't find a wiring diagram for your model but with a lot of microwaves there's a power relay on the control board that is operated to supply power to the high voltage circuit i.e. magnetron, HV capacitor, HV transformer & HV diode and it is released at the end of the cooking cycle, so it could be sticking (stuck?) relay contacts.
This is given that as you say the magnetron is turned off when the door is opened. This is because the primary and monitor door interlock switches have released and operated (respectively) to disconnect the power to the HV circuit and also short circuited the circuit so that it can't operate.
The control board part # is WB56X21278 but it ain't cheap.
You could try locating the relay and just replace it, if you prove that its contacts are permanently operated that is. The relay number should be printed on its case.
crwdns2934105:0crwdne2934105:0
@jayeff You're probably correct. I have the schematic (it's on the inside of the microwave) and it's as you describe. There are multiple relays on the control board, one of which interrupts power to the HV primary. After 10 years it's reasonable that the contacts may have failed closed. Thanks for the control board P/N ($77 + shipping from GE). At this point I have to decide whether I want to put more time & money into repairing an older appliance (that I've always found somewhat annoying to use), or replace it with something more to my liking. Thanks again for your help.
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Hi @ti30xa,
What is the model number of the oven?
Are you sure that the magnetron stays on and not that it is just the turntable or fan still operating i.e. does it still continue heating after the time has elapsed?
What happens after you unplug it and it has stopped and then you reconnect the power and close the door, does it start again by itself?
crwdns2934271:0crwdnd2934271:0 jayeff crwdne2934271:0
@jayeff GE PEM31DF1WW, 800W, manufactured 2013.
It definitely continues heating (food gets hot) but fan and turntable remain off.
If the microwave is unplugged, then reconnected to AC power, the magnetron comes back on, but without turntable or fan.
This isn't the first problem with the microwave. Last summer (2022) one of the door interlock switches (it has three) failed and I replaced it. Given that the microwave is almost 10 years old, it may be time to replace it.
crwdns2934271:0crwdnd2934271:0 Allen Moore crwdne2934271:0