Could it be a cadillac converter plugged - 2005 Cadillac CTS

Edward, Some GM converters have an access plug to remove to, inspect and possibly clean converter, If needs replaced, I would order a quality one on line to replace, Probably half the cost of GM.
Links to converters below.
press the helpful button.
https://www.google.ca/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwil8rbA3f_KAhXDs4MKHQ_mAJsQFgglMAA&url=http%3A%2F%2Fwww.rockauto.com%2Fdbphp%2Fmfr%2CEastern%2520Catalytic%2520Converters&usg=AFQjCNEIyG8UhpZHN66TbLgIaYI293Jr-A&bvm=bv.114195076,d.amc

https://www.google.ca/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0ahUKEwiLmbvb3f_KAhWBtIMKHfVAAJsQFggnMAE&url=http%3A%2F%2Fwww.thewrenchmonkey.ca%2Fauto-parts%2Ftoyota%2Favalon%2F1995%2Fcatalytic-converter%2F&usg=AFQjCNGqTx0pYsnyeqHJo4U1FbwXZnuZ4g&bvm=bv.114195076,d.amc


https://www.google.ca/url?sa=t&rct=j&q=&esrc=s&source=web&cd=4&cad=rja&uact=8&ved=0ahUKEwi57_eC3v_KAhUmkoMKHRTqDJ4QFgg-MAM&url=http%3A%2F%2Fwww.autopartsway.ca%2Ffeaturedpart.cfm%3Fcatalytic-converter&usg=AFQjCNEfrj7NpQZEroaFdoE84R6q73vz6A

How about your cat converter? It may be plugged.

Not the converters. But check them to see if there is nothing in them. I've seen the converters blow apart and clog the muffler. Have the codes read.

Torque converter, torque converter solenoid. both in the transmission. Also the EPC solenoid.

Drop the transmission oil pan, replace filter, reinstall pan. Remove inspection cover to view torque converter. Turn flywheel over by hand till drain plug in converter is at bottom. Remove plug and drain. Replace plug and fill trans. with new fluid.

Plugged fuel filter? Plugged catalytic converter?

Cadillac Northstar owners to those whose vehicle is overheating .My 99 Deville was overheating turns out it was Catalytic Converter.Dont rely on PCM for code.If the Cat converter gets plugged up the heat will not get past converter it will back up into engine block causing overheating.My deville ran at between 190 & 201 degrees this morning.If you have a misfire the unburnt fuel goes into Converter and causes damage to converter (blockage).Alot of people overlook converter when overheating happens.

The Cadillac converters might be bad. I have a 92 and I took the Cadillac converters of and it runs good now.

engine is running rich. Some reasons might be MAF sensor or computer. Is the engine light still on, or what are the codes you got from the previous scans?

the catalytic converter will run for years until the platinum plated beads are old and white in color. when that happens the deposits of the exhaust will build up and plug the converter up making the engines performance very sluggish. eventually the engine will not run until the converter is removed