On the way to work this morning the check engine light came on in my 2002 Hyundai Elantra Manual transmission. At lunch, I went out and had the codes pulled at Autozone. They said it was a P0420 error code. The code is "Catalyst Efficiency Low - Bank 1". In November 2011, I replaced the exhaust manifold, catalytic converter, front exhaust pipe with second catalytic converter & flex pipe, upstream O2 sensor, and downstream O2 sensor. I did all these repairs because the flex joint was leaking then when digging into Elantra issues, I read about the manifolds cracking. My exhaust manifold did have a crack in it. I replaced the O2 sensors because I was replacing everything else and the car had more than 100k miles on it.
What are your thoughts for the possible causes of the P0420 error code? What should I check?