Why would someone do this?
 

I recently bought a 1998 SL2. Seemed to be in very good condition and it seemed to be a good school car for my daughter. The car had been registered out of my county so it was exempt from emissions inspection. When I bought it the MIL was off but came on, on the way home. Had it scanned and it came up PO420. Pulled out the O2 sensors and they checked out fine but I thought something looked funny. Came to realize someone had removed the cat and welded in a straight piece of pipe. I didn't know people still did that. The only reason I can remember anyone ever doing that was so they could run leaded fuel. But you can't even buy that anymore so why remove it. The inside of the pipe is black and sooty but the plugs look new. The downstream O2 sensor was sooty also. I'm wondering if I'm going to have other surprises after I replace the cat. Any ideas?

No idea whats going to come about. Put it back together and see what ya get.