Well known issues in many BMW models, including E39 (the 5-series).

The OBC fancy computer only works if BOTH sender units function properly.
Otherwise, you get all these ghost readings, and you only waste your time playing with these OBC thingies.
Best is to ignore these for now and use the trip odometer as a general guidelines as to when to fill up.

Do the Techron thing, use only good fuel (avoid El Cheapo gas stations).
After a 6-pack of Techron, report back.

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1998 E39 528i 5sp MT
2006 E53 X5 3.0 6sp MT