C3 Picasso fuel lever indicator problem

[ENM]

I inherited a Citroen C3 Picasso first generation (2009) that stood for more than 10 years. So I thought not to start it with old gasoline (I heard it can clog the fuel pump) but not being able to empty the fuel tank I simply put fresh gasoline into the tank over the old one. (It is the 1,4 litre VTi petrol engine.)
Even before all this action I gave contact to the car and the tank indicator showed it is half full. (It does this by showing bars on a scale marked 0-50-100. Those have to be percentages.) This means there were 25 litres in the 50 litres tank.

After I filled in 20 litres of new gasoline from a can (I did not want to put the care in motion for filling it up at the station as I was afraid to clog the gasoline pump) the engine started without problems, but the indicator now shows the tank being completly empty. (Not a single bar.)

Any idea what I have to reset or recalibrate so the indicator shows the real level of petrol in the tank?

[mickthemaverick]

It maybe that the float is holed and when you disturbed it by filling the fuel went into the float and sank it, thereby giving you an empty reading. Easiest check would be to take the unit out hte tank and operate the float manually while watching the guage.