Issue Exhaust i need help

[s10blazerman4x4]

Since cutting off old muffler nothing but trouble.
Had a sporadic SES light for awhile then after last Wednesday and I put on the new muffler it came on for good.Gas mileage took a dump, so i jumped AB and came up with code 13. Changed o2 sensor old one was destroyed removed neg cable to clear code and it cleared code but light stays on.Been checking since Easter and the code is 12 which is system check code. What fuse is the ECM fuse in a 88 cutlass calais 2.5? I got so many not just one that says eCm.

Ok thought it was the cat so we cut it out and it was hollowed out inside and we put in a piece of flex pipe in its place for now but light still on. Is it supposed to be loose up where it connects to the manifold? Like the bolts are tight and there is a spring their but the pipe is all loose.