Toyota Repair: P0420 code on 2000 Sienna, air fuel ratio, oxygen sensor

Question
I know that this code pertains to the catalytic converter, but could be caused by several other problems like a leak, air/fuel sensor or oxygen sensor.  How will  a shop determine the problem and how will I know if their diagnosis is legit so I don't get taken to the cleaners for something I didn't need in the first place?


Answer
The catalityc converter operation can be tested by connecting the diagnostic scantool and looking at the O2 sensor voltage waveforms. The repair shop should be able to show you a printout and explain to you why the catalityc converter should be replaced, A/F ratio sensor and O2 sensor failures will normally set a different code than the 420 so I tend to think that the converter may be the problem, an air leak will adversely affect the fuel trim readings(air/fuel ratio), the technician should be able to interpret these readings and use them to diagnose the problem as well.