Non vanos headers on vanos engine o2 sensor issues

[Romanleal99]

Earlier this year I bought a 6 speed 540i for under 3k, ran like crap because of two bent exhaust valves. Ultimately I pulled the engine and sourced a 60k mile engine with 3 month warranty. My originally cats on the manifold are completely garbage and clogged. I have an extra set of headers and mid pipe off a non vanos 540. Long story short, pre cat o2 sensor harness is WAY too short to reach the connector. Is it safe to cut and solder additional wire to extend it? I know the header swap has been done, but have found no information about the pre o2 sensor location. Any information is appreciated.

[White94RX]

I don't see why lengthening the wires would cause a problem.

[MarcoZandrini]

I used to buy a generic Bosch oxygen sensor for my old 1985 325e. I’d then cut the connector and some of the old cable from the old sensor. I’d then solder the new sensor’s cable onto the old sensor’s connector.

Some tips:
1. Cut the wires old and new wires to different lengths so that the solder joints are staggered.
2. Use shrink tubing for each individual solder joint.
3. Use a lighter to shrink the shrink tubing.
4. Cover the solder joints’ shrink tubing with fabric electrical tape (not plastic tape or shrink tubing) because of the heat.

[Romanleal99]

Lengthening the o2 will change the resistance going through the wires. The sensors work by creating their own voltage, changing the wire length may or may not effect how the engine will run. But there’s no other way around. Im going to have to do it, seems like it’s the cheapest route, besides maybe a tune.

[White94RX]

If done correctly, the increase in resistance will be negligible.


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[Romanleal99]

So what I ended up doing was cutting some of my bad o2 sensor harness (upstream) and cut, spliced, then soldered. No issues with ~800 on the new swap.

Grey goes to Grey, Black goes to Black, and there’s two identical white wires. I found that it doesn’t matter which white wire goes to which. I’ve tested both and read the same exact values on Live data.