Under the filter is a little switch sensor that has a manual reset button in the back. Thinking it is the ALS switch. About size of a quarter, Has two wires coming out, after gong thru the switch. Hangs down in the lower air return and wires run back to main unit above. How do I replace it as it is just DIRTY and dusty. Trying hard to find part number unless have to take apart again to find the number..

With power off it can just be cleaned with a vacuum cleaner or you could replace but you will need the model number to be sure to get the correct one. This is a critical safety part of the unit if it is what I think it is.

There are many 383K model units by Bryant. Below is a copy of one of them. A complete model number is always needed if you are to obtain parts.

http://www.xpedio.carrier.com/idc/groups/public/documents/techlit/om04-3.pdf