[woms_f31]

I wanted to put this together after I looked high and low over a bunch of BMW forums trying to figure this out.

Backstory – After reinstalling my downpipe I made the mistake of not routing the O2 sensor wires where they were supposed to go and melted them on the downpipe.

I know, I know, a really dumb but easy-to-make mistake. Got codes 12B104 and 12B304. Lambda Probe 1 & 2 line break. Ordered new 02 sensors, installed them, and was still getting the codes.

After checking to make sure the wires were connected and ran as they were supposed to be. It got me thinking that maybe a fuse had popped and that was what was causing the new 02 sensors to not be working. The codes did say line break, so the info from the sensors is not getting where it's supposed to go. And the chances of my replacement sensors being bad seemed a bit unlikely. So the bad fuse hunt began. Checked under the hood fuse box, trunk fuse box, and the one by the glove box only to not find a single bad fuse.

Googling “BMW F30 O2 sensor fuse” will bring up a YouTube video just stating that there is a fuse near the DME, and that’s it. Looking under my hood, I couldn’t really find a similar-looking box. Somehow I stumbled on an X5 forum post of a guy opening a Power Distribution Box that contained fuses that could be the source of my problem. Looking closer, I saw that I had a kinda similar box near my DME. Completely different shape, But a similar-looking wire harnesses going in, so I pulled it out to investigate.





Along the bottom edge, you could see a seam. After a lot of fussing about I was finally able to open the box and saw what I needed to see. A circuit board with some blade fuses. But unlike some of the examples I had seen, all my blade fuses were soldered to the board itself. Oh yay. After testing the fuses, I found that indeed one of the fuses was bad. Looking up the boxes part number on Google revealed that a replacement would be around ~$200.



Here is where I should have just bitten the bullet and ordered a new box. But frustration got the best of me, so I pulled out the soldering iron and got to work removing the bad fuse and replacing it with a good fuse. This isn’t easy as the metal the fuse is made out of melts about the same time the solder melts. But I was able to get it replaced with a good fuse.

Pooped the circuit board back in the box, snapped the lid back on, and reinstalled it back next to the DME. Started the car, and no more check engine light, no more wierd idle, and the car was no longer throwing the codes. Success. Some sweat and a lot of fiddling made what would have been a few hundred dollar repair fixable with a soldering iron and a $1 fuse.

Should you attempt to do this? No. Could you do this? I don’t know. But I did want to put the info out there in case someone else makes the same mistake I made, and this fix could be your answer. Forums have helped me with a lot of problems over the years so I'm pumped to be able to give back.

[b58khan]

I have a gen 1 b58 and I also burnt the wires on my post cat o2 sensor, I kept being told there's no fuses for the o2 sensor. This is the only thread I could find for melted o2 sensor. I also wanted to know what the part number is for a gen 1 b58, I have a 2016 bmw 340i rwd.

[Ak1221]

Hey I have the same problem how did you manage to get the fuse out of the circuit board,I tried to pull it out but the pins stayed in ,I’ve soldered the fuse on top but I don’t think that will hold very long I had codes 12B104( oxygen sensor before catalytic converter, heating activation open circuit, and 21A02D (combustion control idle regular) with a Cel after replacing the fuse the Cel went away and so did the 12B104 code but the 21A02D stayed