i bought a z71 a few months back, ive put around 8k miles on it, the guy i bought it from had ran dual exhaust with no cats, (dummy cats for visual effect) and he had removed the aftercat o2s, no engine codes, passed emissions, and now i have a po137 ppo135 code so i replaced the O2s on the drivers side erased the codes and they came back up again. i had assumed that he had deleted out the rear 02s bc i had never had any codes but didnt get a chance to ask him about the o2s when i bought it. any input??