check engine code 51?

My check engine light came on so I tried the ignition on-off trick and got a code 12 (battery disconnected recently) and a code 51 (something about a lean condition). The code 12 I can figure out, but what would cause the code 51. Would it be something I need to worry about or can I just disconnect the battery to clear it and forget it? Thanks in advance.

code 51.......air/fuel at limit. ox. sensor signal input indicates lean air/fuel ratio condition during engine operation.