o2 sensor troubles with aftermarket headers

[S14]

Over the winter I stockpiled a bunch of parts including headers with new gaskets, studs, o2 sensors etc. My car is tuned to have the post cat o2 sensors turned off, so naturally, I bought pre-cat 750mm and 840mm o2 sensors.

750mm - https://www.fcpeuro.com/products/bmw...z3-bosch-13952
840mm - https://www.fcpeuro.com/products/bmw...z3-bosch-13949

Now that I'm fitting everything up, I'm noticing that the 750mm long sensor just isn't long enough. So I hop onto realoem and see that BMW uses same 840mm for cylinders 1-3, but uses a 935mm sensor for cylinders 4-6.

935mm - https://www.fcpeuro.com/products/bmw...z4-bosch-13951

Conveniently, this part cross references to the post cat o2 sensor for cylinders 4-6 on US spec cars.

I've heard that installing o2 sensor backwards will upset can put the car in limp mode, or throw check engine lights. If I hook up 1 pre-cat o2 and the opposite bank post-cat 02, to me, this seems like a recipe for a pissed off DME. What solutions have others come up with?

[Lukem3]

The 935mm is the post cat sensor. Thoes 2 Bosch you bought are the correct ones and should fit no questions asked. Make sure you're not mixing up the bungs and using a post cat bung, and that your wiring is correct

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[Lukem3]

What headers did you get

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[S14]

The 935mm is the post cat sensor. Thoes 2 Bosch you bought are the correct ones and should fit no questions asked. Make sure you're not mixing up the bungs and using a post cat bung, and that your wiring is correct

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I agree with everything you say Luke, which is why I found it odd that the shorter length one doesn't have the length to get to it's connector on top of the valve cover which lead me to looking into Realoem and seeing that the euro exhaust configuration is using the 935mm post cat sensor as a pre-cat sensor! Funny enough, one of the sensors that plugs into the euro section 1 cats is the US spec pre-cat 750mm sensor...

Headers are Megan Racing a.k.a. AA headers, and the two bungs on each bank are right next to each other, I don't see how they could get "mixed up" as both bungs are fed from the same bank. I just double checked and the 840mm sensor is plugged into the first available bung in bank 1 (cyl 1-3) and the 750mm sensor is plugged into the first available bung in bank 2 (cyl 4-6).

[Lukem3]

Can you post pictures of how the wires are run? Are u able to compare the bung location from the stock headers to the Megans? Don't have expierence with their headers, but used their coils on a different car and they were terrible. Could be a manufacturing or design fault. Not sure if the rear sensors differ besides the wire length. Just like I'm not sure why there is the big difference in costs with pre cat bank 1 and bank 2 sensors.

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[Circa Surviven]

I just installed Megan headers and I plugged the post cat 02s with the provided bungs. If your coding it out, I'm not sure it's worth the extra trouble. Even if you decide to install cats later on, I would think not having them in the catless headers would make it easier to run the wire down to euro/supersprint section 1 location.

after re-reading your post are you saying the pre 02s wire isn't long enough?

https://nam3forum.com/forums/forum/main-forum/e46-2001-2006/1739-diy-header-installation

It has the wiring diagram. The pre cat 02s for me reached the valve cover but the wiring doesn't go back as smoothly/well tucked as OEM.

[S14]

I just installed Megan headers and I plugged the post cat 02s with the provided bungs. If your coding it out, I'm not sure it's worth the extra trouble. Even if you decide to install cats later on, I would think not having them in the catless headers would make it easier to run the wire down to euro/supersprint section 1 location.

after re-reading your post are you saying the pre 02s wire isn't long enough?

https://nam3forum.com/forums/forum/main-forum/e46-2001-2006/1739-diy-header-installation

It has the wiring diagram. The pre cat 02s for me reached the valve cover but the wiring doesn't go back as smoothly/well tucked as OEM.

The plug drawings in that link show bank 2 pre-cat and bank 1 post-cat having the same plug head and the bank 1 post-cat sensor is 1035mm, plenty long enough to reach. When I get back out to the garage, I'll pull the long wire off the old headers and give it a test fit.

Are you saying your running all 4 o2 sensors on the megan headers? Does the DME throw a code because there is zero difference now between the pre and post cat readings?

[Circa Surviven]

The plug drawings in that link show bank 2 pre-cat and bank 1 post-cat having the same plug head and the bank 1 post-cat sensor is 1035mm, plenty long enough to reach. When I get back out to the garage, I'll pull the long wire off the old headers and give it a test fit.

Are you saying your running all 4 o2 sensors on the megan headers? Does the DME throw a code because there is zero difference now between the pre and post cat readings?

There is another thread talking about this specifically with guys who know more than me. From reading that thread, I decided to just install the pre cat 02s and put the included "plugs" in the rear sensor spots and hooked up the EGT sensor in the new place on the Megan headers with the EGT wire ziptied out of the way because the wire isn't long enough to go back to it's original spot. I have a CEL for both bank 02s because neither are plugged in on the valve cover. With that being said, from what the other thread says, installing the post cat 02s will still give you a CEL, unless you run some sort of 02 spacer but that still doesn't guarantee success. I plan on getting an HTE tune eventually to get rid of that. I see that you got new sensors, so I'm not sure if you can just label the old ones where each one goes and then hold up the new sensors next to the old ones to visually match them?

[S14]

Just to close up this thread, I ordered a bank 1 post-cat sensor (Bosch 15138) and it plugged right into the bank 2 pre-cat plug with plenty of loom to route the wiring into the valve covers cover without any tension.

[r4dr]

Just to close up this thread, I ordered a bank 1 post-cat sensor (Bosch 15138) and it plugged right into the bank 2 pre-cat plug with plenty of loom to route the wiring into the valve covers cover without any tension.

This might be a stupid question, but is there any difference in the sensors themselves with regard to readings? Are the different part numbers just the same sensor with different lengths of wires?

[jet_dogg]

^and different connectors being a possibility as well

[S14]

This might be a stupid question, but is there any difference in the sensors themselves with regard to readings? Are the different part numbers just the same sensor with different lengths of wires?

I don't believe so, but I honestly dont know. but having a sensor off is a good way to make the DME thing the car is rich/lean and cause a host of issues which is why I chose to order new sensors vs. extending the wiring on a used or to too short sensor.