picked up 2 "front" and 2 "rear" 02 sensors for a 210,000 mile california smog fail 4.0 with high HC (129, passing is 73 at low test, 15 mph) and higher NO levels. Where do they go? Im sure each cat has one before and after it ( I see these) or is the fist one off the manifold? The EGR passages were clean, but the tip of the EGR in the manifold by the TB seemed to not be a turn down, but more straight. Did the turn down tip erode? I think if it were turned down perpendicular to the airflow out the TB that it would create more vacuum. I need more EGR to get these NOx levels down. electric EGR valve stepper was working and was clean. 487ppm is max allowable, I was at 531 ppm at 25mph ( high speed test) but I was using regular gas, nor premium. I think premium would give me less NOx as its recommended for higher compression engines, ie this one? It stated "premium fuel only" on the gas cap. I passed NOx at low speed and HC at high speed..WTH?

 

Found them...2 downstream ones we're easy, 7/8 open end and they were out...of the pipes. Now where the he'll do the harness' go? Under the friggin' console? Yes, you need to pull the console to replace the downstream O2 sensors! Well, nor really. I pulled the carpet back and found you can get your fingers up behind the console and pull the plugs off their clips and pull them down enough to get the plug apart, plug in new one and tuck it back up in the console, put the carpet back and your set. Front ones were a little harder. I ended up taking the plug off the metal clip.and then releasing the plug latch, could not do it when they were attached to clips. 7/8 open end barely fits up in there to break free, you may have to snake the harness into a box end wrench and run the wrench onto the sensor. It works and I got them all replaced in about an hour once the car was up on stands and ramps. I think the rear ones were replaced at one time.as the harness was all twisted, not how lexus ran them off the line. I installed the sensor then plugged it in so I didn't need to pre-twist the sensor.

 

Replaced 4, car started right up and ran great so they work, but it ran great with the old ones too so....I measured all 4 of the old ones and all showed 7.5 ohm. all within spec..:-( so Im selling them used for $40 for all 4. Maybe someone can get a deal on these if they got some bad ones. The pre cat ones mounted on the manifolds were the hardest to take out. a 7/8 crows foot would be perfect tool but a 7/8 wrench works with a lot of positioning. The ones mounted on the exhaust pipes were simple to unscrew but the friggin' harness goes up and UNDER the console! You can get your fingers up in there and unclip the plugs and pull them down so they just clear the bottom of the console without unscrewing anything. Hope these and some premium gas (and some "Pass smog" additive) will help me get over this test.