Lincoln/Mercury Repair: Dashboard \ Fuel codes, mercury cougar, free diagnostic test

Question
I have recently bought a 1989 mercury cougar with digital dash. (I've had the car 1 week)  I  filled the gas tank to a little past the full mark and now the fuel gauge it reads full, but both the top 2 and the bottom 2 lines are lit up.  Then also when I select the "remaining fuel" display it only reads either CO or F.  Toggling between the two.  What does this mean?  Is it just to full to register?
Any information you can give me will be of grat help. Thank You.   S. Braden

Answer
Sharon, this is a good one. It may be that the tank is too full or it could be a bad sensor since the fuel gauge is digital. I would drive it for a while until you know that the tank is not full anymore and if it doesnt stop what its doing then its probally a bad sensor. Take it to a local autozone, and have them do a free diagnostic test to see if the sensor is faulty. Hope this helps.