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Question: ECM problem? Error code PO420.

I bought this van used for a good price and found out why after trying to get it to pass inspection here in MA. The engine light won't stay off.

I took it in to 2 different dealers and both told me that the ECM had to be replaced...for $1300.! They said that the problem is with the logic ("logic has been modified to improve this condition") The part number is 89661-08061 and the new number was ...-08062. Now I find out that there is a newer one ...-08063.

It's out of warranty.

I tried to find one at a junk yard with the new number and haven't been able to yet (most junkyards have just one part number for that model and it doesn't correlate to Toyota's, so I have had to go to each yard and look for myself). I tried a used one with the same number-same problem.

I've talked to re-manufacturers, but you only get to customer service and they have no idea if they are selling an updated version or not.

Do you know if the "logic" issue is a software issue? If it is, can the ECM be reflashed? What is the issue with it?

I know the code suggests an O2 or cat issue, but both dealers I took it to claimed that it was the ECM. Are they right or if I do replace it, will I find I'll have to replace sensors and/or Cat, too.