^What he said.
There's no knock sensors that's why autozone doesn't show 'em, so don't worry about that.
You're brother's truck is probably a little low on oil? If the plenum is blown, it will cause loss of oil, detonation, a bad cat, misfires, lack of power, etc... the new cat will go bad (after a while) if you don't fix the plenum/pcv, if that's the issue. So definitely look into it.