2002 Toyota Camry 2.4 P0420 How do I tell which cat is bad

My 2002 Toyota Camry 2.4 litre. I have a P0420 Catalist efficiency below threshold. No other codes. From what i'm reading here looks like I have a cat gone bad. My problem is that the code reader dosent tell me which cat. There are two cats on my car. One is part of the exhaust manifold and the other after that connected a few feet past the flex pipe.
My budget dosen't have enough in it to buy two cats (barely enough for one). Does anyone know how I can figure out which one it is I have also read that the P0420 can be for a bad O2 sensor. Any truth to that ?

Thank you for any and all help.

yes the cat eficiency code can mean a bad o2 sensor what you need though is a lab scope to watch the voltages in your o2 sensor during operation to determine which o2 sensor it is im not familiar with your car but im assuming there are 4 sensors two upstream from the cats and two downstream from the cats you could also call the dealership and give them your vin and ask them to check for any tsb's related to your problem

Thanks. Dealer says there are 2 o2 sensors. Looks like i'm bringin her in for a full physical at the doctors.

Well the 2.4 is a 4 cylinder engine, on that vehicle there is one O2 (A/F) sensor in front of the cat, one catalytic converter, and one downstream O2 sensor. Most likely you'll need to replace the converter (there is only one), or the rear (downstream) O2 sensor. But as was said, you can diagnose the rear O2 sensor fairly easily with the right tools and know-how. Looks like you've left it to the professionals, they should be able to fix it up right for you.

Thanks.
I wish I were wrong but heres pix of the two cats. They look the same under my hood (just a bit more worn looking).
The first pic is the second cat. The second pic the cat that's part of the exhaust manifold. You can see in the pic the first o2 sensor is before the first cat and the second o2 is after the flex fit. Haven't gotten it in the shop yet. Gettin the $ together. Inspections coming up.

Ahah! I see what you mean now.

You might want to get in touch with the Toyota service dept. Ask about TSB #02404, it may be covered under the extended emissions warranty.

Here's the full TSB: http://www.justanswer.com/uploads/skyvi ... 04_cam.pdf