Cannot resolve check engine light

[rhliberman]

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???

[mmejia]

Well, assuming you didn't try putting leaded gasoline in your car (just kidding) and your oxygen sensors are working fine, I think that you either you still have a leak in your exhaust or you have a faulty coolant temperature sensor.

When you said that your mechanic replaced the catalytic, did he just cut it out and put in another one? Or did he do a direct fit replacement? Because when you do a direct-fit, you have to replace the whole y-pipe from the manifolds and the flex pipe. I would just check the exhaust for any leaks. This is the magnaflow direct fit catalytic for the Avalon:

http://www.car-sound.com/02product/d...irectfit=23880

[rhliberman]

Thanks for your suggestions. Regarding the catalytic, my mechanic replaced the entire exhaust pipe, including the catalytic converter.

[TMack409]

What's the history of the engine? Has it ever overheated? Or otherwise been abused?

[rhliberman]

Originally Posted by TMack409

What's the history of the engine? Has it ever overheated? Or otherwise been abused?

I purchased the car in 2000 with about 20k miles on it. It has been very well-maintained by me. The engine is in excellent shape; no leaks, etc.

Thanks

[TMack409]

try performing a compression test.

FWIW, though I generally perform all of my own maintenance (except for timing belt replacement) - in this case, I would take it to the dealer.

[rhliberman]

Originally Posted by TMack409

try performing a compression test.

FWIW, though I generally perform all of my own maintenance (except for timing belt replacement) - in this case, I would take it to the dealer.

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.