I just changed my primary sensor with a newish used OEM unit and I’m still getting P0135. I’m stubborn and have not purchased new. I’m so over this CEL. I need to bite the bullet and just buy new OEM.

 

You should test for continuity on pins 3 and 4 (the ones with the 2 same coloured wires) on the O2 sensors to see if the sensor is bad.

If the sensors are good, check the harness side of the connector.
I had a P0141 code, which is similar to P0135, but for the heater circuit for the secondary O2 sensor. I tried two sensors only to find that pin 4 on the harness side was receded and not making full contact. I was able to pull it back with a pair of pliers and that solved my problem.

 

By harness side, is this underneath the car where both the O2 sensor plugs are on top of the transmission? How exactly did you see or know it was receded?

I'm struggling with very high negative fuel trim levels, and after replacing the O2 sensor with new OEM the negative fuel trims are now even worse...

 

Yeah, the secondary connector was on the side of the transmission on my MY00 AP1. Once I unclipped it from a metal bracket on the transmission, I was able to easily see it. The primary connector is harder to get to and I had no issues with mine so I didn't attempt taking it apart, however, the diagnosis process of checking for continuity and voltage is the same and is in the shop manual when you search for P0135/P0141.

 

Adding a couple pics to help future generations. If you're like me and you do not want to cut the O2 cable, and you want to have a clean install using the band clips and not zip ties, then to reach one of the bands, use a long, curved needle nose pliers (Harbor Freight has them for a few dollars). I have small hands and still was not able to reach this area with any tool that would help remove it. I tried a slew of tools and methods to get access in there to remove the band cleanly to no avail.

Lay under the car with your feet pointing towards the engine, move your head about a foot to the right and past the Cat then look above it in the crack. Shine a light in there and you will see the band clip holding the cable in place. IUsing the long pliers, and using that picture as reference (zoom in and see the band clip), place the pliers in Front of the Cat in the pic and the pliers go straight up to where the band is clipped, keep the flashlight behind you so you can see in there. The curve of the pliers allows for a perfect angle to grab the bottom of the band clip and either press to release out from the bracket, or like mine it was a bit more brittle and I twisted it til it broke.

 

Thank you very much for this DIY. I just replaced my front (primary) o2 sensor with the Denso 234-4368 (2005 model AP2). Job isn't bad at all. The tip about that plier is money to remove that clip. Then I just a small flat head to unhinge the ziptie to remove it and transfer it over to the new o2 sensor.
My sensor is still working about after 100k+, I'm sure it's not efficient anymore and probably sending wrong signal back to ECU and messing up the a/f ratio. Only around $60 from Advanced Auto Parts store, it's a no brainer.