What does this wire belong to?

[Brewskie]

I recently purchased a 1986 325es auto. While doing a tune up (oil, filter and spark plugs) I noticed this wire was unplugged and figured it was loose and I bumped it causing it to be this way. I plugged the wire back in and went on. After changing the oil and installing the plugs I took her for a spin to make sure all was well. It was not. The car spit and sputtered and wouldn't get above 25mph if I gunned it, it would stop accelerating after 40mph. I pulled off to the side of the road and unplugged the wire and it drove perfectly fine like the day I bought it. What is this wire? Any help would be much appreciated


Edit: the wire is located on the passenger side closest to the fire wall under the hood

[CMatos]

O2 sensor

[Brewskie]

That's what I was thinking but wouldn't it run like crap if it wasn't plugged in not if it was? Even if the o2 sensor is bad?

[CMatos]

You could either unplug it or buy a new O2 sensor and run it properly. FYI, there should also be another identical connector under the car on the passenger side clipped in. You will see that it connects to the O2 sensor.

[CMatos]

If the O2 sensor is bad and giving faulty readings then plugging it in would cause the issues you're describing.

[Brewskie]

The plan is to replace it. Thank you for your help!