Check engine light issue for Toyota Sienna 2001

Help, my check engine light won’t stay off. My mechanic keeps replacing things. He says its got to be the exhaust system. I was wondering if its something electrical or within the car’s computer system. First time he replaced one of the catalytic converters. The light went off then reappeared several days later. He replaced the oxygen sensor. Light off again then back on 2 days later. Then he replaced the other catalytic converter. (I was told there are 3 of them). Once again the engine light went off but reappeared 2 days later. He hasn’t charged me yet since he really hasn’t figured out the problem. I’m taking the van back in next week for yet another look.
Any ideas?..

If there is a CEL (Check Engine Light) there should be one or more codes stored in the car’s computer. The code should be in the formate [P0123]. Do you have that number? If not many auto parts stores will read the codes for you for FREE. Get that number and post it back here. That will tell us a lot.

My experience with Toyotas are that they are happiest with Toyota replacement parts. If the replacements cats were aftermarket, that may be part of the problem. Ditto especially for the oxygen sensor. The OEM Toyota parts cost more, but will fix the problem.

Need codes…only 2 cats are monitered by the PCM. Must be Toyota cats