Hello my name is XXXXX XXXXX I will be more than happy to assist you.
Could you tell me what Year your vehicle is?
There are 3 possibilities; the first one is described in the GM Service Bulletin below. The other two parts that could be the cause is the Fuel Level Sender or the Instrument Cluster.
Start with the bulletin first, if everything checks out then Fuel Level Sender will have to be check out next.
This bulletin is being revised to add 2007 model year. Also, additional information was added to the Important statement. Please discard Corporate Bulletin Number 01-08-45-005D (Section 8 - Body and Accessories).
Some customers may comment on one or more of the following conditions:
These conditions may be intermittent. Upon investigation, the technician may find various DTCs set.
This condition may be due to water leaking into the interior of the vehicle, and/or spread terminals, at connector C305 that is located on the floor to the rear of the driver's seat. This connector is not sealed from the inside of the vehicle and water may enter the connector causing the terminals to become corroded. Also, the female terminals of the connector may have become spread apart causing an intermittent poor connection.
Repair any corroded and/or spread female terminals in connector C305. Refer to Repairing Connector Terminals in the Wiring Systems sub-section of the appropriate service information.
Sorry I didn't notice the picture did not post.