O2 sensor install - lessons learned

[Dispatch20]

This weekend I installed a new fuel filter and 4 new O2 sensors. I think I purchased all of these parts from bimmerparts.com, a forum sponsor. There was recently a great group buy on the NTK/NGK O2 sensors.

The install was straightforward and I learned a few things that would make it quicker if I did it again:
1) No part of the exhaust needs to be dropped
2) 3 of the O2 sensors can be removed with a regular open ended wrench
3) 1 of the pre-cat O2 sensors requires an O2 sensor socket tool that you can get at autozone/pep boys/etc for a few dollars.
4) The pre-cat O2 sensors came out pretty easy, but the post cat were stuck pretty bad. I let the car run for a while to heat up the pipes, and then used a MAPP gas torch on the pipe near the O2 sensor. The heat made removing the sensors a lot easier since I didn't have time for any penetrating oil/liquid wrench.

Once I finished the install I took the car for a spin. It ran AWFUL (horrible mileage, no power, bad idle). It seemed like I had created a massive vacuum leak, and the engines power delivery was awful. I consistently threw the following codes that I read with my Peake code reader:

Table 16

E3 "oxygen sensor adaptation limit, CYL 1-3"
E4 "oxygen sensor adaptation limit, CYL 4-6"

CA "oxygen sensor control limit, CYL 1-3"
CB "oxygen sensor control limit, CYL 4-6"

I searched the forums for what I could have done wrong, and most of the related posts indicated old/dead O2 sensors. However, I had just put brand new ones in. A couple people had suggested that this happens when you switch the connectors around so that CYL 1-3 are connected to CYL 4-6.

I first unplugged the battery overnight to try to clear the cars adaption codes, but this didnt help at all. Next, I swapped the two O2 sensors that are under the plastic cover on the top of the engine and the car ran perfect again. I just wanted to post this up to give any DIY'ers a heads up. I was very careful trying not to switch the O2 sensor plugs, but I still screwed up.

[Balthazarr]

Best thing to do is label them before or immediately after unplugging one at a time.

[NollieStylz]

One of the 02 sensors went out on my car over the weekend, had a shop replace it Monday and the guy was nice enough to let me watch and taught me how to replace the other ones myself. He was very specific when he said to make sure that I don't accidentally switch them around. =P

[NeilM]

Next, I swapped the two O2 sensors that are under the plastic cover on the top of the engine and the car ran perfect again. I just wanted to post this up to give any DIY'ers a heads up. I was very careful trying not to switch the O2 sensor plugs, but I still screwed up.

BTDT! Interesting, isn't it? Happened to me after I did a head gasket R&R on my car.

What happens is that the Bank 1 sensor (cylinders 1-3) will read, let's say, lean. So the ECU calls for some more fuel, the sensor still reads lean, more fuel gets squirted and so on. Meanwhile the reverse is happening on the other bank's sensor. If you clear the adaptation the engine will run fine for a brief period, but the more it adapts the worse it runs.

In my case I assumed (yeah, yeah, I know!) that the forward O2 sensor connector on the fuel rail was for the forward cylinder bank. Seems logical, right? Bzzzt, wrong!

The way it actually works is that since both (in fact all) the O2 sensors have the same harness length, as you snake the wire up around the back of the engine the forward bank has farther to go, so its connector ends up more to the rear of the cyclinder head, and vice versa. That's why the forward connector serves Bank 2 (cyl 4-6) and the rearward connector serves Bank 1.

However due to the different wire routing it works the way you'd expect for the after-cat sensors.

Now what I do is put a paint mark on each end of one of the sensor harnesses so that I don't lose track of which is which once they're pulled up on top of the engine.

Neil
96 M3