Another way to check the P0420 code to see if your converter has lost oxygen storage capability is to scan the catalytic converter oxygen sensor and watch the sensor activity. If it fluctuates between 200 -800 millivolts, the same way as a zirconia oxygen sensor fluctuates, this is pretty good indication oxygen storage capability is gone. OBD 2 monitors the activity between the upstream sensor and the catalytic sensor for this very reason, watching sensor activity. Ensure catalytic converter totally warmed up.
Bill Sullivan
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