The connector under the tank is usually the main culprit in either missing bars on the gauge or the yellow light not coming on.
The connector is rather cheap and sits in an area where it cops lots of bad weather muck.
The tin plating on the male and female ends, get a film of muck and as apparently the current value is rather small, the resistance is too much and the signal doesn't get through.
Each time that has happened to me (bar mising that is) I've used cotton buds, and, industrial Acetone very carefully, so as not to get Acetone on any plastic parts. This, has always worked for me.
One friend scraped his male and female connector with a sharp knife, that worked as well.
Mick.