P0420: How do I know if my catalytic converter is the cause

I got the P0420 CEL code for system efficiency. I want to make sure my cat is actually bad before I put any money down to buy a new one. Are there things I can check or certain symptoms I can look out for? One thing I noticed since I got the CEL is that my truck has had a lower idle, especially when I first start my engine.

Another question, I've recently used Seafoam in my oil and fuel. Do you think the carbon build-up that the Seafoam cleaned is clogging my cat, which is what triggered the CEL?

 

First of all, take it to Advance Auto, have them do a code reset, see if it trips again, could just be a random fluke, especially if you let it sit and idle (happens to me on my Honda Civic). If it trips again relatively soon, would see about having the emissions tested to see what's happening.. Any inspection shop should be able to do that relatively easy.

 

Oh yeah, didn't think about that. It's a sure sign of a bad cat if I don't pass smog

 

a bad cat will sometimes rattle if you smack it with a mallet if the honey comb inside is broken down.

And if you hold your hand over the exhaust and the exhaust is super hot (cant hold your hand over the outlet for more than a few seconds)

These are kinda "unofficial" tests...but can be somewhat of an indicator.