Did you by any chance fill the main tank & whilst doing so try to squeeze in as much as you could, with the sub tank also full? I have found that doing this will cause the sub tank light to remain on every time. Solution is to use some fuel from the main tank, & then disconnect & reconnect the battery. In my case I have already fitted a couple of extra earth cables.

What I have learned to do to avoid the issue to to stop filling when the pump clicks off & to resist the urge to squeeze a bit more in.