The fan turned on randomly and will not shut off

http://www.partselect.com/microwave+replace-fuse+repair.htm

mine turned exhaust fan on automatically... i couldnt shut it off... had to unplug it.... no buttons worked.

Had the same issue, leave Microwave unplugged for 30 minutes and then plug back in. My fax stopped blowing and now fucntions 100% normal again HI/LO / OFF

Best of Luck!!! :)

My Kenmore microwave oven fan has been turning on and off, almost every hour. Sometimes 3 or 4 times in one hour. It's very random. Model No. 721-80593401 Serial No.503TA03208
Please help!!!

The fan will come on if you are cooking below and the area gets hot.... is this when it happens? Sometimes random beep could be a power blip... not long enough for a reset of all things electrical

hi i would think that the power problem has fried the processor
cheers grant

yes the turn table turns, everything works, but no heat.

The vent fan TCO is a normally open type. I would say you replaced it with a normally closed type.

Probably the only TCO that was bad would have been the magnetron TCO.

If you still have the old vent TCO, I'd reinstall it.

Otherwise, you can find helpful exploded view diagrams and order parts by entering your full model number here: http://tinyurl.com/gv383

NOTE: There should also be a "mini-manual" hidden inside the unit behind the control panel or hidden on the left side behind the grille, which is very helpful when troubleshooting.

We're happy to help and we appreciate your thoughtful rating of our answer.

Oh, it turns out the the microwave was over heated and the fan was cooling it. The fan will automatically turn on and stay on until your microwave reaches normal temp. just unplug your microwave and let it cool off. then plug it in and everything should be just fine.

You have likely blown the thermal fuse, its a small silver 1/2" pointy wire on each end and secured by clamp and normally brass crimps to the wires.
usually attached around the fan or motor area.
see this picture
http://chinafuse.en.alibaba.com/product/50391300/50098721/thermal_fuse/Thermal_Fuse/showimg.html