97 protege and a p0420 code

[PGH F4I]

Well I have a 97 protégé that has given me some problems and I am looking for some insight on how to fix this. IT is a 1.5 liter lx model with an automatic transmission and 120K miles. I have a CEL on right now and I had it scanned and I got the P0420 code. I had this code once before and it was a cracked exhaust manifold. It has been almost a year since the manifold was replaced and now the code is back. I have looked at the manifold and I haven't noticed a crack and I don't hear an exhaust leak. Now from my understanding this car has 2 catalytic converters. Now with this code indicating that it has a Catalyst system below threshold (Bank 1) does that indicate that there is some thing wrong with the PRE cat system? I know that the cat for this car from the dealer is ridiculously expensive and I can not find an auto parts store that caries it. Is there some other place that people have found it? I have done some research on here and I have read that this may also be caused by a failing o2 sensor. How does one go about testing the 02 sensors? I did a search here and I could not find a procedure on it.

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[bkey71]

This code is set because the rear oxygen sensor value difference becomes closer to value of the front oxygen sensor. Normally the rear o2 reads pretty steady vs. the front varying constantly rich/lean condition meaning that the cat converters are working correctly. It seems as if the converters are not doing their job.

To get a reading on the o2 sensors, the best way is to hook up the car to a scan tool and observe the values while its running.