Could anyone help me diagnose my Forester? Throwing a code and running lean in boost.

Hey,

I have a 2005 Forester XT, lightly modded and tuned for higher boost.

Currently it throws a CEL with code P0172 - system too rich bank 1.

The other issue is that it's very lean under boost. AFR gauge shows consistent 13:1 under WOT/full boost.

I don't know if these two issues are related. Too rich and too lean are obviously an oxymoron but wonder if one is the truth and the other is ecu compensating for the too rich or too lean condition. I want to rule out anything mechanical before bringing it to my tuner for probably quite a bill.

I checked vacuum hoses and connections near the turbo and boost controller and everything is tight/in order. Don't know where else to look. My fuel pump was used when I bought it, but I couldn't test it myself.

If anyone has any suggestions could you please help me out? If you need any more info I'm happy to provide it. Thanks.

Edit: it's had the same tune for about 4 years now and this is a brand new issue, the code came a few months ago and I only just noticed it running lean under boost last month. Haven't put any new mods on since being tuned. Haven't had the tune adjusted in years. I did take the intercooler off a while ago to work on something but everything looks fine and secured there.

Update: a few days after cleaning the MAF sensor that CEL went away but was still lean in boost and even getting worse. I decided to try just replacing the MAF sensor. That turned out to fix my problem. The car now pulls strong and goes correctly rich under boost.