Originally Posted by Modshack

The reason for this is that the last contact on the sender spends more time uncovered by fuel then any of the others. This gives corrosion a chance to build up. When you fill up, the connection for the last light is not made because of this. As you drive a bit the sloshing fuel moves the contact arm up and down over that last connection. The corrosion is scrubbed off anf the light illuminates. Again, a bottle of Techron concentrate at your next fill-up will go a long way toward keeping these contacts clean..