Toyota Repair: code 420 on ¨01 sienna, mass flow sensor, code 420

Question
Code 420 has reappeared after nine months on ´01 sienna with 115,000 miles.  The first time, this code appeared with codes po446 and po171.  Toyota cleaned evap line and mass flow sensor.  Check engine light went off until now.  Had code checked again and this time only po420.  Toyota tech offers solution of ECU replacement (service bulletin) $1637, or cat converter $1800.  Says they can´t determine which it is, but I can´t spend that kind of money on hit or miss.  Aren´t toyota service shops equipped with diagnostic tools to determine which is the culprit?  Also, delayed shifting and lack of power when cold, but once on highway runs fine.  I also hear what may be air coming out from under the car.  Wouldn´t Toyota have detected an air leak the first time I took it in?  Help!  I want to keep the car for another 100,000, and need to get it fixed without getting taken.  Thanks

Answer
I have read the service bulletin and it staes to replace the ECM with an updated part number, it says nothing about replacing the catalityc converter, so I would recommend replacing the ECM only at this time. By the way, the catalittc converter can be tested by connecting the diagnostic tool and observing the O2 sensor's behaviour, it can also be checked with the diagnostic scantool by looking at the cat monitor test resuslts which the ECM runs automatically, it will show either complete and pass or fail.