Check engine light on. What might it be?

What is your question?

I has my check engine light on for months now. I took it to a garage, the OBD scanner gave a code saying 'catalytic converter'. The mechanic told me it may be the catalytic converter or the o2 sensor. The o2 sensor readings were stuck at the same value(0.41) irrespective of the acceleration. The car runs good and i get decent mileage, but the transmission isn't smooth, it revs up and shifts to the next gear suddenly.