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I have a 2011 Forester 2.5 non-turbo with approximately 100k miles. I've been trying to diagnose a P0171 code (Lean code) and am coming up with nothing. I work in the automotive field and smoke tested the emission system to see if I had a vacuum leak and came up with nothing. I have a CRP123 launch scan tool with which I can see live and freeze frame data and am just wondering the next step to take for the diagnostic process. For some reason, I am leaning towards the bank 1 sensor 1 A/F sensor as the vehicle is approaching 100k miles and I've been experiencing a gradual decrease in MPG. Could anyone please chime in if they had a similar experience or have any words of advice on how to proceed? It would be greatly appreciated. Thank you in advance!
 

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I have a 2011 Forester 2.5 non-turbo with 115k miles and error P0171 that happens every month or so. I reset the code and after a month it comes back. I had a "too lean" error on a 2000 Toyota Corolla that was common, but the devil to find and eventually replaced both oxygen sensors and that seems to have fixed it. What is "the bank 1 sensor 1 A/F" sensor? Is it one of the oxygen sensors? That might be the culprit. The decrease in MPG and hard starting is likely due to a dirty air intake. A good cleaning is what seems to fix that for me.
 
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