i think you should test the lambda first, then you'll know if its faulty without doing a diagnostic. if its faulty replace it, if it isnt then get a scan done.
i know what your next question is, how do you test the lambda? well that information would be here already if i knew what model you have, but you didnt say, and its not in your sig, or on the bravo/a owners list thread.
you could also search fiatforum for "lambda" & "test" and you should find the appropriate method for your model, surely i've posted the lambda test for all models by now.