The issue was discussed a while back of wether it was necessary to pull the pipe up and clean behind the cat. I always do and this is the reason why. I swept the flue with the bypass open, the pic shows all the crap that landed on the bypass and fell in behind the cat. Just pull your pipes up and give it a quick sweep with a vac, it should be easy to do and if its not, it might time to rethink your connector pipe setup.