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P1131 - System lean
You may have a vacuum leak causing this condition or a bad O2 sensor or Mass Air Flow Meter. You will have to hook up a scan tool or oscilloscope and spray the engine down with carb clean while monitoring the O2 conditions to find the vacuum leak. First, check all your vacuum hoses for cracks and your air intake plenum.
This code does not relate to your gas cap at all. That would be a P0441, 442, 455 code in the evaporative emissions system. Yes, you do need one of these tools and the shop that changed your oil should have one of them available unless it is just a quick lube place. They have limited tooling.