The problem with my overhead microwave is that the vent fan doesn't turned off. I could switch back between low and high flow but I could not turn it off. Any help... >

Switch could be faulty/shorted. You are talking about the vent fan that is "under" the microwave, right? In other words, it vents the cooking fumes from your stove below... it's not INSIDE the microwave, right?

Just clarifying. You may be able to replace the switch if it's faulty, just be careful to have the circuit turned OFF before messing with any wires in there.

Hope that helps.

Yes, I am talking about the vent fan that is "under" the microwave.Any hint how to replace that switch? Thanks in advance.

Hi,
You need to know the model name and number of the vent or equipment; not really the name of the switch.

After you determine what it's called with the model number; Like Sears, Model #... ; you can do a search on Google for it.
Best wishes,
fredg

To check or replace the switch, you must open up the housing where it is. And yes, turn the power off first. First make sure none of the wires to it are shorted to each other. If it is a simple switch independent of the microwave, you should be able to pick up one from a hardware or Radio Shack. If it is integrated into the microwave controls, you will need to through the manufacturer. You may be able to pick one up from a local dealer or appliance repair service.

A number of years ago, I replaced a complicated, integrated switch for the blower and light on my range hood with a couple of switches from RS.