SES Light on 02 530iA (P0313, P0154, P0174)

SES Light has been off and on since I bought the car 9 months ago. Usually, I would get just the P0174 code and then clear it after a few days. Only, that same code would come back after a month or so. This last time however, it has been on for a month and then it would go off on its own for a day or so, then come back on. I got a code scanner and pulled these three codes. I just got done messing with it, hoping that its just an intake leak. I hooked up a smoke machine to the intake and found nothing after the MAF sensor. A buddy thought it might be an exhaust leak, since one of the codes is an O2 sensor. So, we hooked up the smoke machine up again, but nothing visible is coming out. Any other ideas? The light has been cleared again so, I guess I will have to see when the light comes back on.

P0313
P0154
P0174

Thanks,
James

SES Light on 02 530iA (P0313, P0154, P0174)