I have a 99 Chevy Silverado 5.3l Z71 and my MPG's have dropped from a best of 17.5mpg/avg of 15.5mpg to the low 14's the last 2 tanks. In Oct last year it started throwing the P0171 and P0174 codes. The truck still ran fine and I was still averaging 15.5mpg after clearing the codes. Near the end of Oct before a hunting trip, the code came back and I replaced the pre-cat O2 sensors(Bosch) and during the 2 round trips from Yuma to Tucson I had the best fuel mileage I have ever had with this truck during the month of Nov with no codes. Then of course the first week of December I get the same codes back so I ordered new AC Delco intake gaskets and AC Delco knock sensors and wiring harness and replaced them. Ever since I replaced those parts along with cleaning the MAF and throttle body, I am getting the crappiest fuel mileage I have ever had since I owned the truck. I checked all my spark plugs(AC Delco 41-110) the other night and they all looked good and put a bottle of Redline SI-1 fuel system cleaner in this current tank and right now I'm just a hair off of a 1/4 tank left and 250 mile on the odo. The truck idles and runs good it seems, but just crappy fuel mileage. Could the Bosch O2 sensors be the cause even though they seemed to be good in the beginning? If so, I might get AC Delco O2 sensors and take the Bosch's back for a refund. Thanks in advance for any help/suggestions.