Threestangs

My check engine light came on two days ago. Prior to the light coming on, I noticed that my RPMs stay fairly high for 2-3 seconds even after I engage the clutch. I hooked it up to a hand held engine code reader and the P1131 (O2 Sensor) code came up. Appears that the engine is running lean. Also, I have learned that there are a million potential reasons for the P1131 code to pop up.

Any ideas? I have heard that the IAC tend to go bad after a while. I have 86K on the car and it is completely stock (no mods).

Thanks
Chris