99 audi a6 gas problems
 

can anyone help me?
I'm having a problem with my gas gauge...
i cant let my car get below a quarter tank without the check engine light coming on and the engine stalling....
so i always fill up before that point... so i end up putting 13 gallons in everytime to fill up, seeing i have an 18 gallon tank, theres 5 gallons I just cant access... it cant get to the point where the gas light comes on, because i cant use those five gallons, the check engine light comes on and im screwed, its a pain in the butt fill ing up every thirteen gallons, and it sucks knowing something is wrong... can some one help me please?!