Toyota Repair: 2000 Tacoma code P0420....again, otc scanner, catalyst system

Question
Ted, the Toyota dealer replaced my cat and one 02 sensor, forward of the cat, under error code P0420 one month ago. The truck had 79,800 and the parts were covered under warrenty. Now at 81K, the same error code is back. Question, could the rear 02 sensor be bad? I have an OTC scanner but do not have the specs for the sensor. Should the dealer have replaced both sensors while they had the first time? Any chance they will repair this as its the same error or am I up the creek?  

Answer
If the rear O2 sensor had failed it would give specific code for the sensor, the 420 code is strictly a cat converter code and specifically states catalyst system performance below threshold, I would recommend going back to the delaer and have them check the cat to see if it's operating correctly this can be done with the diagnostic scantool and should be done at no charge to you.