Just got an evo 9 gsr about a month ago, get cel for running lean P0171 about once a week. have checked hoses, i/c piping for leaks, no obvious problems. clear it out and runs nice all the rest of the time, no other codes, pulls strong, no other known issues. a/f gauge states 15.0 while cruising slight lean? goes down to about 10.0 on full throttle/boost, but blanks out (over 17.0) when coasting are these numbers normal? any ideas?

mods: ams intercooler, mishimoto radiator, hks manual boost control knob turned down to factory (peaks at about 17lbs boost holds strong), injen intake, fuel clinic 1000cc injectors, headers and catback exhaust, A/F gauge, turbo gauge, greddy boost controller (set to off for now), and a whine from the rear think prob has a fuel pump. not sure if any others just got it...

 

Numbers are typical.... I have seen cars run 17 at cruise without issue... Youre front o2 might be giving up the ghost

 

probably bad o2 sensor