Hoping to get some expert advice here if you all would be so kind. Here's all the information. My baby, which I love, is sick, 2002 WRX sedan, 55000 miles, everything stock never modded, and it's out of warranty since August.
Problems began October 27th, MIL came on with a DTC of P0171 (System Too Lean). Power was questionable (hesitation/erratic) under hard acceleration in any gear, particularly in a spot between 4000-4500 rpm. Opening the throttle up in a higher gear (say 4th) seemed to be the trigger that caused the ECU to report the P0171. MIL came on several times with P0171 and I cleared it several times. Couple of those times the EVAP monitor was set to incomplete as well.
I tried a couple of light maintenance remedies. Replaced the Air Filter because the old one was pretty dirty. Checked the MAF sensor for dirt, found it to be clean. Put a bottle of fuel injector cleaner in a fresh tank of gas. These things probably retrieved some lost power but didn't resolve the symptoms.
November 6th I took it into the dealership for diagnosis. The mechanic there took the snapshot out of the OBD, checked all the solenoids out ok, checked the fuel pressure ok. I asked him to do a vacuum leak test because I suspected a leak somewhere in the intake perhaps downstream of the MAF, but he didn't do this. Instead Subaru told him to replace the bank 1 heated O2 sensor in the downpipe and reflash the ECM to a newer version and reset it. Special ordered the O2 sensor part. Went in yesterday (November 16th) and had it done along with a LOF for good measure.
Initially power seemed more smooth and the MIL wasn't coming back on, and I was feeling good. But by the end of the night yesterday coming back up the freeway, on goes the MIL, P0171 again, this time with the EVAP and O2 SENSOR monitors set incomplete. And the power is again erratic in the 4000-4500 rpm range. Right where the boost starts to be felt. So I realized I just spent the cash out of pocket for the O2 sensor replacement part and labor, and the problem isn't resolved. #%$&!
Before I go back and raise hell with the service department at the dealership, I want to find out what you guys think. My inclination is to tell them to do a vacuum leak test like I asked for in the first place. Maybe it's just a loose clamp or cracked air duct somewhere, you know, something less severe. Maybe the turbo is dead, something more severe, I just don't know. TIA