I had the code P0135 in the ECU of my 99, last week I replaced the 1st 02 sensor in the front bank (closest to radiator) and the code went away. Today 8 days later the CE light came back on and the code P0135 is back again. What gives?

 

Well did you get your code reset or cleared? If you just left it after replacing the O2, then it probably doesn't know that you actually replaced the sensor. The code goes away over a period of time even if you do not replace the part.

 

I did clear the code after I replaced the sensor, the light stayed off for 8 days before it came back on today.

 

Are you sure you changed the correct sensor? I thought that Bank 1 (aka Right bank), was cylinders 1,3,5. Since the car is Front wheel drive the front of the engine is facing your passenger side. When reffering to parts on your car you're sitting behind the drivers seat. If you were to make believe the car was Rear wheel, turn the motor so the timing chain is facing forward. That would make cylinder 1,3,5, on the right side if you were sitting in the driver's seat. That would make P0135 (Right Bank) O2 sensor located towards the firewall. Make sense or am I just

 

Not 100% sure I got the correct sensor, but the light did go out for 8 days after I replaced it.

 

Your best bet is to change the other sensor to be safe I have a feeling that you might have changed the wrong sensor. I'm interested in what happens keep me posted. Thanks

 

Wait a minute, you said you cleared the code, now in the quote you said it went out, MAKE UP YOUR MIND!

 

I replaced the sensor, and cleared the code which turned the light off for 8 days, today the light came back on and the code is back.

 

change all o2 sensor before cat ...

 

Yeah I agree, just b/c the light went out doesn't mean that you changed the right sensor. Some people have their lights go out even when they change NO sensor.