Issues with the B Button and Shutdown

[PaladinX87]

Well I took it back apart and cleaned the contacts on the cm3 board itself, and while it looks like that fixed up the issue with the start and Y buttons, the B button was still dead. I will give it a shot cleaning the socket contacts tomorrow. So dumb question, I have a multi-meter but I've only used it for my riding lawn mower, what/how would I use it on the CM3?

Thanks again for all of your suggestion thus far, pushed me to try somethings I am not familiar with