My 06 Ultra gas guage reads up and then down but only on full does it read correctly. The low fuel light does work when it should. Sending unit or guage?

 

I'd think it more likely to be the sending unit than the guage.
If you have the FSM there may be testing procedures to determine where the problem lies and how to test the sender.

 

My needle started bouncing back and forth one day, it was the gauge. When you turn the key on the gauge should slowly rise to the level, not jump.

 

When first turned on the guage slowly goes up to the level but when riding it goes down then up and at reserve it reads about half a tank.

 

Mine on the glide never worked right.fuel light did and the miles until empty were fine.

 

Test the sending unit.