P0420 code, Much needed help!!

[Cruisergirl1]

I have a 2006 touring edition PT cruiser with 108,000. My check engine light came on and I took it into orileys and the "P0420" code came up. I took it to get a diagnostic test and they gave me a huge list of things that need attending but that my main cause was the P0420 code. I've had the car for a month and I don’t know what has/hasn’t been changed. I also have no dealership warranty. So I feel like this is going to be expensive. I'm already overwhelmed. :|

I have been reading up on it and from what I understand it’s pretty much my catalytic converter needs to be replaced or it’s my 02 oxygen sensor. I want to know how can I tell which needs to be replaced? I have a muffler shop looking at it as we speak. I don’t want to be taken for a fool. I understand more than likely they will say I need my whole catalytic converter replaced because it’s the more expensive of the two.

Also, I have been experiencing problems when I'm idling. The car shakes and does like a "jump". When it does that my A/C gets hot. Once I resume driving my A/C gets cold again. The check engine light has never come on before until last week so I stopped driving it and took it in to get looked at.

[CREWZIN]

Something to read....

P0420--Cat efficiency low bank 1?