I recently replaced my clutch on my car and while I had my stock y pipe off I had a friend fix and exhaust leak I had. The leak was caused by a crack going 3/4 of the way around my pipe just after the upstream o2 sensor on the passenger side. I was driving around town and my check engine light came on. I took it to a shop and they said it was a P0420 code and that it was most likely an o2 sensor. My car isn't running funny and the exhaust sounds fine other than a leak I can hear on the drivers side manifold. I just need to know if that sounds like the problem or if I should go to a shop with a better scanner and see if they can pinpoint the problem better.