Have you tried looking at the Fan description here - 18.104.22.168.22.214.171.124.126.96.36.199.2
This is what I see when testing this OID on a 3845 - Power Supply 1 Fan
I see no way of getting the status of Power Supply 2....
okay, least I know i'm not crazy now. I was monitoring the 1.2 that you were speaking of after realizing that the other always reported fine. Chalk up the fan 2 thing to another wonderful cisco product haha.