Here is my problem. I have 1997 ES300 with 102k miles. Had a check engine light go off, code P0135 (oxygen sensor 1 bank 1), I replaced the sensor closes to the firewall before the car. Was good for about 200 miles and light came back on again. Now I have code p0135 and p0420. I know 0420 is catlayst below threshold. But why p0135 again? Is it on because p0420 is on? I know p0420 is triggered via the rear o2 sensor (post cat). Should I replace this along with the cat? I just don't wanna spend money replacing stuff that is fine in order to track down this problem. I may just install an o2 simulator on the post cat sensor to avoid heavy repair costs. Any ideas or suggestions

 

Youll need a scan tool to see the activity of the o2 sensors in a perfect world. But if you cant do that check the wiring, replace all the o2 sensors with OE parts since 100K is the norm for those anyway and clear the codes. If they come back Id try getting the pcm reflashed before you throw a cat in it. But check the wiring for that bank 1 sensor. I really dont think you can fool the pcm with a dummy o2 sensor either.

 

ok so you have a good 02 sensor that works well but the other one which is just used to montior cat efficiency really stands out as a loser so the ecu says you might have a cat issue or a post cat sensor issue. if i were you, i would either test the post cat sensor or replace it. if problem returns you would seem to have a cat issue.

 

A break in the wiring or the pcm not grounding the bank 1 circuit would seem more likely than a bad cat to me. hint - (corrosion)

 

Isn't bank 1 sensor 1 the very first o2 sensor that you see when you open the hood? The one in front of the engine?

 

No bank 1 is the rear.