Nissan: 01 Frontier Catalytic Converter Bad?, snap on modis, obd ii codes

Question
I recently bought a 2001 Nissan Frontier 2.4L 4 cyl with almost 100,000 miles.  Within 3 days I had a Service Engine Soon light.  I pulled the OBD-II codes that showed  Catalyst System Fault.  I ran a scope on the front and back O2 sensors, and the front one modulates as expected as soon as the engine starts, and the back one started modulating in about 2-3 minutes.  I understand the back one shouldn't really modulate at all if the cat is working well.  When I did an underneath inspection, I noticed that there isn't any sensor after what I believe to be the cat, about halfway down the exhaust.  There is one just in front (upstream) of the cat. The entire tailpipe looks like it is original, but it may not be.  I'm wondering if that is the normal configuration, or if someone put a pre-1996/ODB-II exhaust system on the truck?  Also, if I just change out the cat with this configuration, I don't know if I will fix my problem.  Here in CA, the car should have passed smog inspection prior to the sale, but I didn't see any paperwork when I got it from the corner dealer.  

Answer
If someone removed an 02 sensor you would have a hard code for it and not a cat code. Nissan cats go bad more often than any others. Connect a good scope to the vehicle like a Snap on Modis or the OEM scanner and watch the 02s in action while you are driving. If they read the same or even close then your Cat is shot or on its way. You also have to look at your fuel trims and you MAF. What is the system trying to do to compensate? Usually the computer will try and change things to correct the problem it sees. The data streams will allow you to analyze what its doing and point you in the right direction