maxmac

I have a bone stock 04.5 2500 6-speed with 280,000 miles on it. Recently it has begun to have rough idle at cold start as well as an overall power loss - just does not get up and go like it used to. Turbo appears to spool up fine, no leaks, etc.
Tailpipe fell off as a result of cracked outlet. Replacing muffler and tailpipe this weekend. Wondering if maybe the cat is clogged??? Had this happen on a gasser in the past.
Can't check codes because my odometer does not illuminate anymore. Any ideas to help troubleshoot both the rough idle and the power loss????