[carwhat]

Hello,

I don't know much about cars, but I hope this is the right place to ask a question about a problem I'm having. I did search and found something similar, but not exactly the same as what I need.

Today, my check engine light came on. (The car is a 2000 Toyota Corolla.) After bringing it to my mechanic, I was told that it was the code for the catalytic converter. He said that it could possibly be the oxygen sensor, so since he would replace both anyway, I should try replacing the sensor first, and see if that alone would clear the engine light. After asking him a few questions, he mentioned that for now my only problem would be that I wouldn't pass a vehicle emissions test. Funny thing is, I actually just took my car for the emissions test two days ago and it passed. Does that mean that it is not the catalytic converter, or is it possible that it was ok then and suddenly two days later, it needs to be replaced? I figured if it weren't working or would be on it's way towards needing to be replaced, I probably would not have passed emissions, is that right?

I then brought the car to a second mechanic. He said that the first guy doesn't know what he's talking about, because if it were the sensor the code would have said so. He said he wouldn't replace the sensor at all, but just replace the converter. However, like I said, I did pass the emissions test two days ago.

They each wanted to charge between $260 to 3 or 4 hundred dollars for the converter. However, I went to auto zone and found a universal one for my car for $80. The guy there did a diagnostic reading too and it said:

P0420 - Catalyst system efficiency below threshold - bank 1. Probable cause:

1. Air leak in exhaust before rear h02s (heated oxygen sensor) bank 1
2. AF sensor error
3. Fuel system fault
4. Faulty catalytic converter

Some of the customers there gave other suggestions of what the problem could be such as needing to seal the gas tank better or something like that? I am not completely sure.

Either way, I am not really sure what to do. I don't want to buy parts and pay for labor so that the mechanics can keep trying things to decide what it is that is making the engine light go on, and they each keep giving conflicting opinions. I am going to go to a third mechanic in the morning, and maybe the Toyota dealer, but does anyone have any advice as to what it could be? Does the fact that I just passed emissions two days ago make any difference?

Thanks in advance!