My truck had a fairly major cat failure, but never set a catalyst efficiency code. In fact, it never set a code at all.
It was acting like it couldn't breathe. Performance was fine below 2500 RPM, then it would kind of fall on its face, then above 3000 RPM it would flash the CEL indicating a misfire, but would not set a code. I could hear major detonation/misfiring going on at these engine speeds. Went over the ignition system and fuel system and nothing was going on there...had to be airflow related.
One day on the highway it started misfiring, I reached over and touched the floorboard...it was HOT. Like really freaking hot. The truck had four converters (LEV, default CA emissions)...I pulled the rear two and ran a serp belt tool through them. I tried to get the two up front, but that was a no-go due to the bends in the pipe. I ended up ordering a Magnaflow y-pipe. Unfortunately the Magnaflow stainless y-pipe looked good, but did not fit the truck at all. Simply impossible to install it on a automatic trans truck. I swapped it for a Walker, and the Walker dropped right in like a glove.
7 months later, no catalyst efficiency code. No CEL at all in fact. Apparently the two Walker pre-cats are doing the job. No misfires, no detonation, no catalyst efficiency code...just a smooth running, slightly louder truck. I fully expected a P0420, but it has not happened.
Not really proud of my "fix" for the rear converters, and may eventually put new ones on as the truck does drone and attracts unwanted attention, but I was quick fixing and when all performance problems went away and there was no CEL, I kind of stopped. I am myself shocked that the CEL has not gone off at all with two rear cats completely gutted. The front two seem to be doing a good job of cleaning, or this old Vulcan just doesn't pass too many nasties through the exhaust somehow. Go figure.
BTW - Tried Mr. Gasket Cataclean and it did nothing.