I have had the "lean bank 2" error code for some time. In addition, have had the check engine light come on and go off many times over the past 2 months or so. Have replaced, both 02 sensors (upstream) both pcv valves, MAF sensor, purge valve, evap canister, new gas cap and even had a fuel injector "cleaning" at my local GM dealer. Car runs fine, there are no shakes or rough idle, nothing. My brother in law was visiting from Texas (he is a heavy equip. diesel mech.) He connected his laptop scanner that he uses for work and after checking out a lot of things, he believes there is a air leak somewhere "un-mitigated" air as he put it. Highly recommended smoking the engine to find it. The mechanic I deal with does have a smoke machine and I asked them to do it, but I don't think they did. Have thrown over $600 at this problem and so far, no solution.  please help .....