When you see 1/2 as was mentioned, the first number indicates bank one or driver side. Second number indicates the downstream or after cat o2 sensor. So your code tells you to look at the drivers side downstream sensor.

If you had simultaneous codes, 1/2 and 2/2, then I would suspect air leak at T Body. With only one code, I would suspect sensor. Any air leak after the MAF sensor will give impression of lean condition for both banks.