Fuel reserve light stay on

[NaidenM]

Hi all
Please help, i dtive a 2004 citi 1.4i, my fuel reserve light stay on, i replaced the sender unit in the tank, fuelpump relay, even the cluster, we traced the wiring aswell, got the fuel gauge to work for a day, keep in mind, tank is full and when i got it to work, but when you turn the ignition on the gauge work, as soon as you start the car, gauge move to empty and reserve light an buzzer come on.
Please advise

[Belix]

I'm certainly no expert on electronics or Citis, but would think that the sender would likely use a variable resistor - depending on the fuel level, it would give a reading between zero amps at full resistance and full current at no resistance. As for whether full tank is full resistance or the other way round I don't know.
The point being that if the wiring is either shorting somewhere along the way, or has an open circuit, it would override the actual reading.