I have a 2006 mustang GT. Mainly stock except for a BBK CAI and corsa axle back exhaust. I also have a oil catch can. I have had lean bank 1 P0171 and bank 2 P0174 both coming on for several months now. I had a shop that i trust check for vacuum leaks and they found none. I have replaced the MAF and the fuel filter and the codes still come on. The shop thinks i am getting too much air and suggested getting a tune or put the old OEM air filter system back on. Can anyone help me diagnose this problem? Vehicle is located in canada near Detroit.

Thanks.

 

Without knowing the I.D. of the GT airbox, or the BBK, I would tend to agree with the shop. I will try to explain without getting into unnecessary details...
In simple terms, your MAF measures airflow based on the diameter of the tube it is mounted in, and the speed of the air moving over the sensor. Most (but not all) CAIs have a larger-than-stock tube diameter. Computers are not very smart, and can only do what they are programmed to do. Since the inside diameter at the MAF is one of the parameters programmed into the computer, changing that I.D. without changing the program ( "getting a tune" ), will freak the computer out as it attempts, and fails, to correct the problem.

If you have the ability to measure the inside diameter of both tubes (as close as possible to the MAF), start there. If they don't measure exactly the same, put your stock system back on until you can get a proper tune for the BBK. If they do measure the same, your problem may lie somewhere else.