Fan does not activate when cooking

There is a wire mesh filter and a carbon filter on the under side of the unit. IF they are plugged then it wont draw air. The other issue is this unit has a dual option for installing the fan. One is to vent to the outside and the other is to vent to the inside, if the fan is not installed for the correct usage, you have no airflow either.

It is not the fault with the fan but the trigger to the fan is shorted. It can be the DOOR SWITCH or the Transistor switch from the board. A fault or error on the processor cannot be ruled out. Clean the main board and reset.

I had GE look at the problem today (my microwave is less than a year old and under warranty). He replaced the cam louver and that seems to have fixed the problem. This is a small motor accessible by removing three screws and the active vent door. The whole visit was less than 15 minutes.<!-- from fixya rte -->

Interesting post, because I have the same problem! I have spent the better part of today researching and taking our microwave apart (partially) to inspect the top portion re: outside vent. Everytime the microwave is on, the "active vent door" (manufacturer's term, not mine!) opens. Apparently this is normal. But it also opens when you turn on the vent fan, when cooking on the stovetop. I don't like this feature, because I think it 'overrides' the outside venting system. I double checked our outside vent to ensure that when this oven was installed, the person did in fact allow the flap to be open when the fan is on (he did). So I think what 'our' problem is, is that the microwave active vent sort of interferes with the outside vent, thus more of the cooking smell being blown into the house instead of the outdoors. I was thinking of simply disconnecting the gizmo that make the active vent door open each time you use the oven and/or vent fan option however I don't think that's a good idea as apparently the main purpose of the active vent door is to keep the electronics of the microwave oven from overheating. The more I read about this version of microwave, the less impressed I am with it. Lots of electronic issues with it. I am not fussy either with having the microwave over the cooktop. Sorry I don't have a solution for your problem but I do feel your pain!!!

Yes, this microwave will vent either out through the top or the back. A damper will be needed if it was not saved when the unit was installed. A list of other items are listed in the installation instructions at this link To GE.
http://products.geappliances.com/MarketingObjectRetrieval/Dispatcher?RequestType=PDF&Name=49-40409-1.PDF

Hi. It sounds to me like the control PCB was damaged and is not switching power to the cooling fan or T/T motor.... Does the oven light work?

That is normal for it to spin for about a second or so becuase of switch changeover but should not remain on constantly when door is open. Have it checked up if it does not happen like the way I described it.

A simple test is to place a piece of bread in the microwave ( it heats up very fast) and then if your microwave was actually ON and generating RF, the bread would get hot very fast.. If it remains cool then only the fan is on. It appears that you may have a faulty door switch thou because once you open the door, your microwave should be off..

Are you sure the VENT is not on? That blower will operate regardless of the door open or closed so you must switch it OFF.

what is the make and model number

It may be a bad door switch, a loose door switch mount, or a problem on the control panel, such as a sticky relay.

I would suspect a sticky relay first, but the door switches should be checked.

If you or a friend decide to look into it, we have critical information on safety, disassembly and door switches at our site, linked here on Fixya: http://tinyurl.com/yzjozk

If you need your control panel assembly (smart board) repaired, we do that for $39.95 nationwide.

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