My truck is missing at around 2krpm. The only code that comes up is P0420. Can a clogged cat cause a misfire, or is the other way around? I took it to the dealership and they said it is the cat. They want way to much money for the part + labor. They said they did a back pressure check and that leads to the cat being bad. Please help, I dont want to dump money in the wrong parts. 99 silverado with 4.3 and 101k