So hopefully this forum is a little more helpful the one that will remain nameless. I have a 2006 S4 with 87,000 miles on it and its all stock. The original problem was no power and I was having codes for P0171 Lean on bank 1, and P 0305, 0307, 0308 Misfires in 5, 7, and 8, So I replaced all the plugs air filter. Still had no power and code P0171 for bank 1 but no misfire codes. so I swapped Ing coils for bank 1 to bank 2 to see if it would follow and it didn’t. So I move on to replacing the PCV valve, do a carbon clean, swap inj. from bank 1 to 2, and install intake spacers. While the Intake was off I noticed the valley pan was leaking so I replaced the gasket as well. After getting all that done I do not have any P0171 code but now have a P0431 Bank 2 cat not operating with in threshold. and when I accelerate fast it builds up the speed slowly and makes this sound I have a hard time explaining. Checked for an exhaust leak and only really saw one at the slip flange between the main cat and muffler on bank 2 side but it seems really small if it actually is a leak( just see the black carbon build up) was going to pull the down pipes off and check the cats and hollowing out the pre cats but if anyone has the trick and what tools work the best for this id greatly appreciate a how to. Anyways I thought maybe my mass air flow sensor was bad to I disconnected it to see if the car would get its power back and it didn’t. So I rechecked my vacuum lines and found one vacuum line broke that went to a silverish regulator behind the oil filter housing (there is one on the other side as well) I fixed it and now I don’t get any codes but the power loss is still there. Im at a loss right now any help would be awesome I miss driving my car. Oh and I don’t know if this maters but when revving the eng. it doesn’t make the sound only under load.
