car running lean?

well the intake gasket went fine, everything seems all good and the injectors seem to be working out great. day after i finished it though, the starter solenoid and armature went bad and caused no start (yeah i was freaking when i got the phone call). but fixed it in a day. since then car has started up way too quick (guess cant complain). you just turn it to the start position, and it starts in an instant. car has also had much greater response then ever, gas mileage not bad for all city being 23-26.

but now, just yesterday my mom comes back from picking up pizza and walks over to the manual. i ask whats wrong and she said the service engine soon light was on. i told her she wouldnt find anything in the manual yet she keeps looking till she finds out my right. so simply ask has it been acting weird, running off? nope nothing wrong. my first though was O2 sensor or MAF. so today i had it checked and i didnt get to see the codes, for some odd reason, but i caught a glimse that said PO171, and im certain i saw more but couldnt say what they were.

well after seeing the PO171 i figured i was dead on, but now which am i going with? well the car has 150K or something with stock O2's so i figure replaces O2's anyways, and im going to clean the MAF. the only thing that made me mad was finding out this v6 doesnt have oxygen sensors in both banks (only 1 before cat usually v's have 2), so i cant exactly narrow it down.

another thing to add, once warm and i start it it idles horrible like a vacuum leak, yet after i move and stop again i dont think it happens again. so im going to check vacuum hoses but i doubt its the problem, and clean the maf tonight. ive ordered the O2's, so im going to replace those tomorrow.

ok i checked codes again, and this time i was shown the codes. P0171, P0135, and P0171 again. so today ive got to pick up the O2 sensors and replace them. after seeing that second code im certain its the O2 now.