Why the vent fan won't turn off

bam out of PC to ovens.
in 1 click.

I would say you replaced a good hood thermostat (or "TCO" / thermal cutout) with a new thermostat of the wrong type.
The oven cavity TCO and the magnetron TCO will read zero ohms continuity when tested.
They are "normally closed" devices.
But the hood vent exhaust fan's TCO is a "normally open" TCO or thermostat, so it should read open or "infinity" on a continuity check.
If the temperature under the microwave gets too high, the hood TCO opens, which turns on the vent fan until it cools back down.
I would bet that only your fuse was blown.

It could be that the fan button is sticking or a contact has welded itself closed therefore its getting power all the time.

the fan motor needs tobe turned around take screws out of top front remove front fan grill pull out fan assibliy and turn around. this will pull air out of your house

Your microwave turning on by itself is a "self preservation feature" of the GE Profile Advantium and maybe some other models. Read the manual, When your over the range oven gets warm enough, for whatever reason the venting fan comes on until a satisfactory oven temperature is reached. You are not permitted to shut the vent fan off. Also it seems a temperature near room temp must be achieved before it shuts the fan motor off, it is very annoying. We had to return our Advantium because of fan noise. I configured the ovens vent fan to the recirculate direction because we do not have a vent duct that exits the house over our range. When the vent fan acting as the ovens cooling fan automatically turned on we would have to leave the house to hear each other talk. Too bad we really liked the oven other than that major problem. G-Man

Right, that would be MCO10UW and the owner's manual is here.

The owner's manual does say this, of course,

"To prevent overheating from taking place, the exhaust
fan is designed to automatically turn on at a low speed if
excessive temperatures occur. Should this happen, the
fan cannot be manually turned off, but it will
automatically turn off when the internal parts have
cooled. The fan may stay on for up to an hour after the
range and microwave oven controls have been turned
off."

That;'s standard on microwaves.

But these models do have a separate internal cooling fan (which should run any time you microwave) plus the hood vent exhaust fan, which should only run when you select it, or if there is an excessive stovetop or under-hood temperature.

So if it comes on whenever you microwave, that is incorrect.

When that happens, it's usually a defective vent thermal switch (a.k.a. hood TCO).

But since it's brand new, I would certainly contact Magic Chef about this. If it's in need of repair, they will fix it free, assuming the oven wasn't physically damaged.

For more help from Magic Chef / Ewave see here or their contact page.

For Magic Chef in Canada call 866.587.2002

We're happy to help you with free advice and we'd appreciate your thoughtful rating of our answer.

You can find your owner's manual online here.

That should help you clarify & verify proper operation.

If you still need help, you can contact Jenn-Air (Maytag) customer service here.

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

which fan? the exhaust fan or the control board cooling fan? the cooling fan is controlled by a thermostat just behind the control panel. it turns the fan to protect the board from overheating.

SOME BUT NOT ALL OVENS HAVE RELAYS TO TURN THINGS ON AND OFF CHECK THE OVEN TO SEE IF YOU HAVE ANY
RELAYS AND SEE WHAT THEY DO.

From looking at the owner's manual, it looks like the vent fan is only supposed to come on when the internal temperature of the microwave gets too high OR when you press the VENT FAN pad. Here are a couple of fan-related excerpts from the owner's manual, which can be found here: http://products.geappliances.com/ProdContent/Dispatcher?REQUEST=SKUOBJECTS&DOCUMENT=Use%20and%20Care%20Manual&SKU=JVM1840BD (or the better URL here: http://tinyurl.com/yn7r6g) Automatic Fan An automatic fan feature protects the microwave from too much heat rising from the cooktop below it. It automatically turns on if it senses too much heat. If you have turned the fan on, you may find that you cannot turn it off. The fan will automatically turn off when the internal parts are cool. It may stay on for 30 minutes or more after the cooktop and microwave controls are turned off. - Vent Fan The vent fan removes steam and other vapors from surface cooking. Press VENT FAN once for high fan speed, twice for medium fan speed, a third time for low faw speed, or a fourth time to turn the fan off. Press the VENT FAN BOOST pad for extra fan speed. - Hope this helps.