There is fairly comprehensive trouble-shooting guidance in the FAQ too.
https://www.volvoclub.org.uk/faq/Ele...augeFluctuates
There should be a multi-plug connector in the luggage compartment area which makes it reasonably simple to interrupt the circuit to the sender for testing purposes. There are four wires in this connector. The thinner ones are for the fuel gauge and the thicker ones for the in-tank pump.
Getting the instrument cluster out on a 940 is quite simple. Accessing the fuel sender a bit less so. This may influence the order in which you test components.
Good luck.