I need some help trying to figure out what is going on with my 98 Avalon XLS. Here's the history:

08/05/08: Check engine light came on. Took to mechanic who found a code P0135 - heater circuit bank sensor - replaced air fuel ratio sensor. He also noticed that the exhaust pipe had a leak so he replaced the pipe/catalytic converter.

Within a few days the engine light back came back on.

08/29/08: Took to mechanic. Code P0420. He checked the catalytic converter and all oxygen sensors - all checked fine. He cleared the code.

Within a few days the engine light back came back on.

10/07/08: Back to mechanic. Code P0420 again. The new catalytic converter had a rattle so he replaced it under warranty.

Within a few days the engine light back came back on.

11/19/08: Back to mechanic. Codes P1410 & P1411. Replaced EGR valve position sensor.

11/20/08: Engine light came back on. Back to mechanic. Now it's showing a Code P0420 again. I did not leave the car with him. He told me he had one more idea about what is causing the problem but did not want to say what it was yet. I told him I wanted to do some research before I bring it back.

I've been using this mechanic for about a year and have been very satisfied with the server. I believe this is just a weird fluke.

On the other hand, I keep replacing parts and the problem keeps returning.

Any ideas???

Take it to the dealer. They generally don't practice the art of Remove & Replace to get things fixed.

Thanks for your suggestion, but I've never had a good experience at a dealer - even a Toyota dealer. My mechanic specializes in Toyotas and I think my issue is just a fluke; I was hoping that perhaps somebody had a similar experience and could share their resolution.