RE: 97 protege and a p0420 code
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.