Turned fan on low walked away. fan turned on high by its self and will not turn off

quite likely your microwave is trying to prevent it self from self destruct, because if the fan is not keeping the electronics components cool enough while its running the high limit thermo will disconnect power from the heating circuit, if the fan is not working it should be replaced , that's a techs job.

I’m happy to assist further over the phone at https://www.6ya.com/expert/phillip_77d32603f8052503

Fan works when component heat is higher so its normal.

There may be a faulty door switch, " among the group" that will need to be tested to find the culprit.

There are typically 2 fans in these units. The fan that cools the magnetron and then the vent fan that intakes from the top of your stove then vents the air out or away from the unit. Are the lights that shine on the top of the stove working?? Sometimes if a bulb blows violently it can blow a trace off of the back of the main circuit board and that trace may also supply power to the fan

NOT THAT I'M AWARE OF . YOUR UNIT SHOULD HAVE A SWITCH TO TURN ON AND MUST BE TURNED ON MANUALLY

Does this oven work okay ,but the fan start when ever it wants?Can you tell me if it comes on when someone is walking around near it.No has to touch it.

no, it is not controlled by a thermostat

the fan has to stay on,thats the way they are made

check the control unit and high voltage circuit, and megnetron, high voltage diodes, wiring and high voltage fuse etc.,

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.