Need Help; Engine service light (code PO 171) system running lean left bank. This light comes on and then goes off after a few restart cycles. dealer has replaced, Oxy sensor, MAF sensor, dropped the fuel tank to check for trash. We may drive car for several miles before light comes back on. Dealer can not or has not found problem because it is a intermittent problem and has not done it while he has Envoy. Problem occurs and goes away but has set code. Envoy will not pass inspection with the code set. PLEASE COMMENT ON THIS PROBLEM IF YOU HAVE ANY SUGGESTIONS. THANKS