Service Engine Light Comes Back. Do I Need New O2 Sensor?

[TrafficJam]

About 3 weeks ago I had multiple lights on my dash. I got both TA and several speed sensor replaced at the shop. The shop owner didn't replace the O2 sensor at that moment, because he want to see if it is a real bad O2 sensor or some other factors.

Previous Thread:
[Expert Needed!] Do I need to replace my throttle actuators?
https://www.m3post.com/forums/showthread.php?t=1949252

I brought the car to the track the next day. Right before I arrived, the service engine light / increased emissions message came back again with the following error code presented:
27C7: Lambda probe adaptation trim control bank 2
I asked the shop owner and he said bank 2 (driver side) O2 sensor is bad and need a replacement. Since those are just faulty sensors, I plan to DIY. Here are three questions I want to ask:

1. Does replacing the O2 sensor solves the problem (I hope it is not related to any other issue such as exhaust leak) ?
2. From my understanding, there are 2 sensors (pre-cat / post-cat) for each bank. Do I need to replace both sensors on the driver side (bank 2) ?
3. I will use this link as DIY tutorial:

do you guys think it is reliable?

Note: Car is bone stock.


Thanks everyone for helping!

[MaximusB]

Swap sensors from bank 2 to 1. If the code jumps over to bank 1, then it is a bad sensor. If code stays at bank 2 than likely exhaust leak at gasket before the cat.

If bad sensor just replace the one, no need to replace all of them unless you want to.

[NiBe]

2C7C is postcat O2 sensor. Btw my tip is exhaust leak. I had this error on my E91 and it was exhaust leak. My understanding is that the error does not say that there is problem with O2 sensor but it says that DME is trying to compensate non optimal air/fuel mixture and it is not helping (typically because of exhaust leak aka extra air in exhaust) so it tries to compensate more and more until it reaches a limit and throws error.

I would smoke test your exhaust. In a shop or you can DYI. I recommend this method (tried them all and only this one worked).

- create a smoke machine via some YT video
- connect vacuum cleaner to the exhaust tip and close the other tips
- smoke around suspected areas and if there is a hole you will see smoke being sucked to the exhaust

If you are thinking why not to smoke directly into the exhaust then it is because you would need to have a big hole in exhaust to see smoke via this method. If there is a small hole (was my case) then this method does not work well.

Keep us updated about progress.

Good reading on this topic: https://bimmerprofs.com/n52b30-probl...ulating-probe/

[TrafficJam]

Originally Posted by MaximusB

Swap sensors from bank 2 to 1. If the code jumps over to bank 1, then it is a bad sensor. If code stays at bank 2 than likely exhaust leak at gasket before the cat.

If bad sensor just replace the one, no need to replace all of them unless you want to.

This is a good idea, I will try. Thanks!

[TrafficJam]

Originally Posted by NiBe

2C7C is postcat O2 sensor. Btw my tip is exhaust leak. I had this error on my E91 and it was exhaust leak. My understanding is that the error does not say that there is problem with O2 sensor but it says that DME is trying to compensate non optimal air/fuel mixture and it is not helping (typically because of exhaust leak aka extra air in exhaust) so it tries to compensate more and more until it reaches a limit and throws error.

I would smoke test your exhaust. In a shop or you can DYI. I recommend this method (tried them all and only this one worked).

- create a smoke machine via some YT video
- connect vacuum cleaner to the exhaust tip and close the other tips
- smoke around suspected areas and if there is a hole you will see smoke being sucked to the exhaust

If you are thinking why not to smoke directly into the exhaust then it is because you would need to have a big hole in exhaust to see smoke via this method. If there is a small hole (was my case) then this method does not work well.

Keep us updated about progress.

Good reading on this topic: https://bimmerprofs.com/n52b30-probl...ulating-probe/

This is a great idea, I will try. I did the rod bearings and TA at a bmw specify indy-shop and he never mentioned my car has exhaust leak. I will confirm with him again as well. I will certainly keep update, and hope this is just a sensor issue (since fixing an exhaust leak sounds harder to DIY lol).

[TrafficJam]

Originally Posted by NiBe

2C7C is postcat O2 sensor. Btw my tip is exhaust leak. I had this error on my E91 and it was exhaust leak. My understanding is that the error does not say that there is problem with O2 sensor but it says that DME is trying to compensate non optimal air/fuel mixture and it is not helping (typically because of exhaust leak aka extra air in exhaust) so it tries to compensate more and more until it reaches a limit and throws error.

I would smoke test your exhaust. In a shop or you can DYI. I recommend this method (tried them all and only this one worked).

- create a smoke machine via some YT video
- connect vacuum cleaner to the exhaust tip and close the other tips
- smoke around suspected areas and if there is a hole you will see smoke being sucked to the exhaust

If you are thinking why not to smoke directly into the exhaust then it is because you would need to have a big hole in exhaust to see smoke via this method. If there is a small hole (was my case) then this method does not work well.

Keep us updated about progress.

Good reading on this topic: https://bimmerprofs.com/n52b30-probl...ulating-probe/

Updates: It is not an exhaust leak, just a failed pre-cat O2 sensor. I purchase both pre-cat & post-cat sensor, but end up only replaced the pre-cat (post-cat sensor was too tight so i gave up at that moment). I did a 100 miles round trip during the weekend, runs 7 track sessions, and the light didn't come back. I guess the problem is solved and I will just return the post-cat sensor lol.

[NiBe]

Great! Hope that the outcome will be the same after 1000 miles then I would believe that it is indeed sorted out.