2002 Buick Lesabre

2002 Buick Lesabre 6 cyl Front Wheel Drive Automatic 880000 miles

My fuel guage on my LaSabre is inaccurate. The guage will indicate a full tank until 3/8 is comsumed. The guage will start to come off the full indicator but is still inaccurate. This condition has persisted since I had a Buick dealership trouble shoot and repair " Check Engine" condition. What can be done to correct this condition or do I have to replace the sensor.
Michael Rodcay
April 8, 2008.

Either there is a gauge problem, a ground problem, wiring problem or a problem in the tank, such as a dented tank or a sending unit problem.