Catalytic converter

My Rodeo has been throwing the P0420 code for a long time. Usually I turn the CEL off and it stays off for a while. Now gas mileage has become an issue. I think it may be time for a new cat, any thoughts? Should I try the O2 sensor first, or just go after the cat? My 96 is a manual transmission, with a single cat on the drivers side. While I am not looking forward to replacing this, I have had the cat assembly off recently to replace the starter, so I am aware of what is in front of me.

If anybody has any thoughts on a good (read less $) replacement cat, let me know. I can do a bolt on, or weld on, or even the entire assembly if that makes sense.

Thanks for any help.

Hey, kind of off the subject, noticed that you run 15w-40 in your Rodeo; notice any difference on power?

I have not noticed any increase in power, but it seems to consume less oil. Even if it does not make any difference in anything, it is great oil.

Yeah, you definitely will not see an *increase* in power; was wondering actually if you've noticed a *decrease* in power?

I run Maxlife 10w-30 and currently have 170k miles; must be one of the lucky owners, mine only uses maybe 1/8qt every 3k miles or so.