if your petrol light works, i think fuelgauge itself is broken. you need a multimeter (volt-ampere-ohmmeter) to check if it works. it`s easy job, if you know a bit about electricity.

but i think you first should check that fuel sensor works. you test whit your multimeter how many ohms the sensor shows when its Empty and how many ohms when full. then slowly move sensor from empty to full, and watch that ohms rise and lowers smoothly, as you move fuelsensor. then if it works, you put sensor in one position, and look how many ohms there is. then you check if there is same amount of ohms, in your fuelgauge. if there is same amount of ohms, you have broken fuelgauge. buy a new one.

note, you have to turn cars power on, when checkin fuelgauge.