How to determine faulty o2 sensor? 12b404

[Nicky_D]

Hi all,

I'm getting a 12b404 code on my F80 M3, after searching it comes back as the following "12B404 - oxygen sensor heating after catalytic converter 2, actuation: line disconnection". This leads me to believe one of my secondary o2 sensors has failed but I am trying to determine which sensor it is as there are two downpipes each with a secondary sensor. Has anyone come across this code before/has a way to pinpoint the faulty one?

Thank you, Nick

[Djdnav]

Part number 11787848487

[GP0256]

If it's secondary it's downstream. Do you have downpipes or anything where the wires could have been burned?

[Nicky_D]

Originally Posted by GP0256

If it's secondary it's downstream. Do you have downpipes or anything where the wires could have been burned?

I do have VRSF downpipes but was running a burble tune with high aggression. I think that caused the sensor to fail. I have since replaced both downstream sensors and the CEL is gone. Didnt look like any of the wires were damaged but thank you for the input!

[paulo_f80m3]

This just happened to me it looks like but with the post bank 1 sensor. I'm on stock downpipes and I just replaced mine which just failed again after 300miles... Could the exhaust burble really cause this? Has the CEL come back?

[Nicky_D]

Originally Posted by paulo_f80m3

This just happened to me it looks like but with the post bank 1 sensor. I'm on stock downpipes and I just replaced mine which just failed again after 300miles... Could the exhaust burble really cause this? Has the CEL come back?

The CEL is now gone, it was stored as a permanent code but cleared itself after a driving cycle. I'm not certain it was the tune but I don't think anything else would have caused it. The car had 48k when the failure occurred. If you buy the sensors on fcp euro they are guaranteed for life. When one breaks ship it back and they credit your account. These are not cheap sensors which I found out the hard way after initially ordering the wrong one. Each rear sensor has a different plug. One is yellow and the other blue. Good luck and let me know if you have any questions around this. Bosch is a good oe replacement but those were all sold out when I was looking so oem it is. Also I'm catless so I had the cold start feature turned off but switched it back on after replacing these. That might have had something to do with it but again I'm not sure unfortunately

[paulo_f80m3]

Originally Posted by Nicky_D

The CEL is now gone, it was stored as a permanent code but cleared itself after a driving cycle. I'm not certain it was the tune but I don't think anything else would have caused it. The car had 48k when the failure occurred. If you buy the sensors on fcp euro they are guaranteed for life. When one breaks ship it back and they credit your account. These are not cheap sensors which I found out the hard way after initially ordering the wrong one. Each rear sensor has a different plug. One is yellow and the other blue. Good luck and let me know if you have any questions around this. Bosch is a good oe replacement but those were all sold out when I was looking so oem it is. Also I'm catless so I had the cold start feature turned off but switched it back on after replacing these. That might have had something to do with it but again I'm not sure unfortunately

I replaced the sensor yesterday and once I removed it I noticed some condensation in the sensor. It's not fuel so that's good. I have a weird feeling that it's the low rpm cold start that's affecting this. I feel like it's not letting the condensation push out and it just sits in the sensor. Maybe I'm wrong but I'll still have to test the factory start for a couple of hundred miles and if the problem doesn't come back then that's the issue. Maybe my car just doesn't like it. Went with the OE Bosch sensor you mentioned from FCP Euro.