Toyota Repair: 420 trouble code, voltage waveforms, exhaust leaks

Question
QUESTION: My 2000 Toyota corolla 1.8 has a recurring 420 trouble code I have change the front oxygen sensor and the rear oxygen sensor and the catalytic converter twice I don't know what else to do can you help ?

ANSWER: The Po420 code is usually because of an inefficient catalityc converter but it could also be caused by an exhaust leak either at the exhaust manifold or the front pipe.

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QUESTION: I have change the catalityc converter twice and there are no exhaust leaks any where.whaat must I do next?

Answer
Instead of replacing more parts I would have a technician that can connect a diagnostic tool do a catalityc converter test, this is done by comparing voltage waveforms of both the O2 sensors which will determine the efficiency of the converter. Then go from there, if the voltage readings are normal then the ECM may have to be replaced to solve this.