i am new here but work on cars for a living....im 20 and had my chevy for almost a year now. when i first got it it was great very powerful, seemed to get good gas mileage and i loved it. after like 2 weeks or so the engine light came on and the code read fuel trim lean bank 1 and bank 2. took it to the dealer, where i bought it, and they said it was the o2 sensors so they replaced the front o2 sensors and for another week everything was fine til the light came back on with the same codes. i tood it back and they replaced the o2 sensor after the cat and the light stayed off after that. first question is i dont think the rear o2 sensor is used to determine fuel trim,am i right? next i found a vacuum leak in the 4x4 lines to the front differential, fixed that and changed the fuel filter. but since then i have been averaging about 17 miles to the gallon with a higher flowing exhaust, also seems like less power. is there something that the dealer could have done to make it like dump more fuel in the cylinders or something to hurt my gas mileage but not making the engine light come on to get the lean code to go away? light does work

 

Well I know for sure the Front O2 sensors read and adjust fuel trim. The rear O2 sensors read the condition of the cat.

 

Saw the exact same thing on the same truck, kept putting o2 sensors in and the light kept coming back on. More than likely youve got a bad cat but there expensive, so get whats called a o2 sensor simulator you hook em up and it tricks youre commputer to think that everythings good. It sends electric impulses to your sensors. google it buy one there pretty cool

 

b4 trying 2 fool the system with an aftermarket gizmo like KawaChev10 suggested, why not determine what the problem really is, instead of guessing what it might b. if it is the cat, they're not that expensive 2 replace. u can buy a universal cat and have it installed for around $100. since this would actually solve the problem (if this is indeed the problem), your truck would run/perform better-and most importantly, not use excess fuel for no reason. with the current cost of fuel, i say this was a pretty good reason to fix the problem, not hide it.