Camry sensors replaced; light back on

I may not have enough details for anyone to offer help here, but I have to try.

I have a 2006 Camry LE (4 cyl.) which got a new (factory) catalytic converter in early September, the one mounted on the front of the engine (the car has 94,000 miles). I replaced it because it had an after-market on it when I bought it that had no shields. I decided to ante up and get a new cat so it could be properly covered. In about a month the check engine light came on. My mechanic said it was an 02 sensor. He changed the downstream one that has connections running up into the car under the seat. The light came back on. He changed the other downstream sensor. The light came back on. Getting a 0 volts reading on the last sensor he put in, he put another one in. The light came back on. Because I was away for a week, further work was delayed until today. The diagnostic today wasn’t conclusive; I think he said the codes were 136 and pending 137. He thought it looked like the air/fuel sensor was the culprit. So that was changed. The light came back on tonight.

He said this morning before that sensor was replaced that it could be the computer. Has anyone had this kind of thing happen who might think it’s the computer? What else could it be? I know there are other variables (bad connections and contamination, for example), but he checked what could be seen. Is it time to take it to a Toyota dealer?

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My mechanic checked the computer again this morning when I went into get part numbers for the Toyota dealership. In short, their report showed nothing wrong with the sensors even though it showed a 137 code. They cleared the code and drove it, tested it again, and it showed no code. I’m suspicious of the no-code thing. Yesterday after the computer was cleared, it took about 40 minutes of driving before the check engine light came back on. I don’t think their 10 minute driving this morning was long enough. The only things left to do is check the #2 sensor directly for voltage and do a smoke test on the exhaust. It was too hot to do that today. The service writer said that if the light came on, I should put a piece of tape over it and come back in after my road trip for the tests just mentioned.