I have a 2001 S-80 2.9l on turbo that has two ck engine codes , Fuel too lean. I found the following what looks like a check valve in the air intake manifold hose on the left side on the manifold intake. This part has a hole in it and I think that is where I'm sucking in air.

 

This part is the power brake booster check valve. I had a hole in it which was on the under side of the valve. I had to rotate it to find the hole. I believe that is where the air is being sucked into the intake and is causing my P0171 and p0174 " bank 2 fuel too lean engine codes. I will replace this and report back if that clears the codes.