The fuel gauge will not work but it's full of petrol, also the fuel low light stays on , changed the sender but no change, any ideas what's the problem? It's a 1.4  petrol Fabia 2002 

Could well be a failed solder joint on the back of the instrument cluster, where the sender connects. 

If I'm looking at the right diagram then it appears pin 15 at the instrument panel and pin 3 at the sender should be connected. Pin 3 should also show a voltage (5 volt I'd wager) when unplugged. There should be connection between ground and pin 2.

 

Ground is probably fine if everything else is working as expected. It wouldn't hurt to check resistance between the pins as the fuel level drops to ensure sender functionality, new stuff can be bad out of the box.

Edited 3 October, 20178 yr by LightRain

The gauge moves slightly , to top of red only, but light stays on all the time 

if you have miles left on the digital and it works, then it's prob'ly the cluster/guage