P2A00 code, HELP

raps101

I'm getting this code. Pa200 bank 1 sensor 1 (air/fuel ratio sensor). Changed the sensor with a brand new one from the Nissan dealer parts department/Nissan oem, code came back a few days later. Cleared it, and it came back again a few days later. Mechanic says he performed a smoke test to check for a vacuum leak. No dice. Someone else's 2 cents says this code is common on Nissans and he suggests there may be a small crack or cracks in the exhaust manifold causing leaks, or nuts broken/missing connecting the exhaust manifold to the engine. Anyone have any thoughts?

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