The P0420 DTC means that the catalytic converter is below a minimum efficiency level set by the PCM. To put it in simple terms, the stupid torn intake pipe just ruined my catalytic converter which in the 2009-2011 cars is $1800 from the dealer! (Because it's a Maniverter; the exhaust manifold and catalytic converter is one big unit)
It's unlikely that the broken intake hose caused your cat convertor to go bad. (I've had split intake hoses on most of the Asian cars I've owned and it never caused the cat to fail.) I recently changed the flex pipe on my '09. The cat converter is NOT in the manifold. It is in the first pipe that is farther back, rearward of the flex pipe. But beware - changing the flex pipe is a bitch of a job, because the flange connection of the two pipes becomes galvanized into one solid mass of rust. That means that if your cat is bad, you will need to replace the front flex pipe as well.