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The P0455 indicates a "large leak" in the evaporative emissions system. Yes, it could be related to a gas cap, but it could also be many other things, which I think is the case.
The P0171 indicates the engine is running lean. Basically, that means there is too much air getting to the engine and not enough fuel. Since you have both codes, I feel there is a vacuum leak related which is causing both problems.
What I need you to do is check the EVAP systems hoses and charcoal canister. I attached two pictures below. The first shows the EVAP hose routing diagram. The second is a legend which identifies the parts in picture 1.
Check those things. Also, if you have replaced the fuel cap already and the code is still present, confirm the filler tube between where you add gas and the fuel tank isn't damaged, rusted, or have any holes.
Let me know what you find,
Images (Click to enlarge)
Monday, April 8th, 2019 AT 7:05 PM