P0171 code, engine knock, and VTC sensor

[gosmithpa]

Had my 2001 Pathfinder (100,700 miles) scanned at Advance Auto. P0171 System Too Lean (Bank 1) code found. Nissan service rep. felt that code with engine knock was VTC Sensor. Replaced VTC sensor, 23731-2Y524 on driver's side of engine. Severe engine knock went away immediately. Thought problem was solved. Went to Advance to turn off SES light yesterday. Everything was fine for about one day until SES light went on again on the way home from work. Stopped at Advance again only to find the code read was the same again - P0171. Inspection due on 8/31? What is the next step to troubleshoot? Replace the same sensor on passenger's side?