I have a 2005 Jeep Grand Cherokee that has a P0420 code which has recently shown up. I am not sure how long the heater has not been working as I live in GA and have just begun to start using it but the heat coming out is mildly warm. Finally, after warming up, the idle for from 1000 RPM to around 500RPM which I believe to be too low.
I know that the cat could be bad, especially given that it had a blown head gasket which was fixed earlier this year but at the same time, I think there is also a possibility the code is from something else given the other symptoms I'm experiencing.
Edit: The hose into the heater core is hot but the hose coming out is just warm. Not cold, however, so I believe coolant is flowing to the core, just not much.