1998 Mercury Mystique Lean Codes

About a month ago we were having problems with the engine staying a idle. When the codes came up, I had them checked and got P1507, Idle Air Controller. I replaced that part but in the process of repair (it was cold outside) I found that two of the vacuum lines were broken. One was for sure my fault, but the other I am not sure. Nonetheless, I replaced the two vacuum lines and the car started fine with no check engine light. About two weeks ago the light came back on. Today, my girlfriend took it to the auto parts store to get the code read and they said, "Lean Mixture on both banks" and didn't give her the code numbers. My question is should I start with the replacing the other vacuum lines or should I continue with your diagnostic videos (section 2) on your page about Lean mixture.