Catalyst Monitor (Good Catalyst)
WARNING: This page is about a different car, the 2003 Pontiac Vibe. However, it is still accessible from the selected car via links, so may be relevant.
A good TWC catalyst will show a very active output voltage on the pre-catalyst heated oxygen sensor. A good catalyst, 95 percent hydrocarbon conversion, will show a relatively flat output voltage on the post-catalyst heated oxygen sensor. See Figure .