Fuel Gauge show only half a tank after a fill up

[z3aficionado]

Hey Z4 Gurus, my 2003 Z4 only shows a half tank after I did a fill up. I did read up on some threads about this and I have a few questions.

Could this possibly be a matter of reprogramming the gas gauge and not a fuel sensor problem?
I read there are two fuel lever sensors. One is behind the passenger seat. How do you get to this one and what exactly does it do?
I also read the other one is built into the gas tank and that is not easy to get to. Again what exactly does this one do?

What are your thoughts of using Red Line fuel system cleaner to try to clean up the sensors?

Any help is appreciated.

[jclausen]

It's probably one of the two fuel level sensors scan the car with an auto logic they're are probably faults present