The CEL came on this week out of the blue, no modifications, no changes in weather, nothing new I haven't done before.

I'm getting a code 67.
"Catalyst system Efficiency Below Threshold malfunctioning or defective catalyst system"
I searched a few pages back on it, and all the threads I saw were the result of either an aftermarket manifold or cat. I have neither, so I'm wondering what I can do before I hae to resort to having someone look at it.

I have one extra o2 sensor on hand, so I can check my sensors tomorrow and change the dirtiest one, reset the battery and check for a CEL. Any other ideas? What are my chances that it's actually the cat?

Also, with 135k miles on this 96 Civic, I'm wondering how the extended emissions warranty I've heard about applies to me (a non-Californian)?