jfmorris

Interesting. Does sound like they had to redo the exhaust for some reason in the past, but unless it is restricted (i.e. too small a diameter from the point the pipes merge), not sure why that would cause heat to build, without causing other performance issues due to too much back pressure in the exhaust.

I think at this point I've got no other ideas to help, but am still glad the aux fan made a difference.

vms4evr

I have a 2006 Silverado 1500 w/5.3L flex fuel. It came from the factory with 2 cats, 1 on each manifold. The 2 cats are part of a Y-pipe that goes to the passenger side. Then runs back as a single larger tube to a big muffler under the bed and out the side. So single exhaust with dual cats. The cats are fairly small and welded to the y-pipe so it acts as a single piece. It flows fine and is very quiet.
I have since done a cat back that replaces the piping and muffler starting behind where the y-pipe becomes a single tube. Bigger piping and free flow muffler. Louder but not obnoxious. Torque has improved quite a bit. Pulls much better off the line.
Stock and modified my coolant temps are always consistent.

I would venture to guess that if you have a plugged cat your engine temps are going to go up. Both oil and coolant temps. I've dealt with that with my 04 CTS-V having bad cats and running hot as hell. Replaced cats and all was good. It was also throwing codes because the O2 sensors upstream and downstream were not happy.