this happens at around 30 kph and goes away once i accelerate.

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This post was auto-generated based upon a question asked on our tech article page here: Pelican Technical Article: Replacing your 3-Series E30 Oxygen (Lambda) Sensor

I assume because you are in the O2 sensor forum you are talking about a check engine light on with O2 sensor code that goes away under acceleration. Does the O2 sensor have 4 wires? That means it has a heater built into it. O2 sensors work best when they are hot and exhaust heats them up. At idle they can cool down so the heater is supposed to keep it hot. Make sure your heater circuit has the power and ground it needs to work.

- Nick