The CTS is unique, in that it has two sending units due to tank design. The fuel level is calculated at the IPC(I think). I would have the codes checked, and gauges tested with a scan tool prior to doing anything. The tank removal is about a 3-4 hour repair. Here is a bulletin that gives a couple of scenarios for example: #PI01969: Runs Out of Fuel When Gauge Reads 1/4 Tank, Possible DTC P1172 - kw 3.2 connector DIC display electrical fluctuation inaccurate LA3 low MIL P0445 P0453 P0461 P0463 P2066 P2068 pump tank stall - (Apr 9, 2004)
Subject: Runs Out of Fuel When Gauge Reads 1/4 Tank, Possible DTC P1172
When servicing the vehicle with the VIN you entered, the following diagnosis might be helpful if the vehicle exhibits the described symptoms.
Condition 1: Fuel gauge goes to E, runs out of fuel when gauge indicates 1/4 tank, possible DTC P1172. Upon removal of fuel tank technician may find left tank saddle has fuel in it and right tank saddle is empty.
Cause: Inoperative, plugged, or cracked jet pump. The jet pump is integral to the primary (right) fuel module and transfers fuel from the secondary (left) tank to the primary (right) tank. The jet pump plastic boss changed 9/02 to address the cracking issue. The weld equipment for the jet pump was changed 11/22/02 to address plastic weld flash plugging the jet pump.
Correction: Replace primary (right or filler-neck side) fuel module. The labor operation is L1200, the part number is 25679320. The date code for the primary fuel module is on the label on top of the unit, stamped into the upper right corner in the following format: 1MMDDY.
Condition 2: Fuel gauge reads empty after a fuel fill with the engine running. Both DTC P0461 Stuck Primary Fuel Level Sensor Signal and DTC P2066 Stuck Secondary Fuel Level Sensor Signal set together.
Cause: This can be customer-induced by fueling the vehicle with engine running.
Correction: Diagnose the system per published Service Information procedures. If no concerns are found, clear the codes to reset the PCM and to restore gauge function. Do not replace parts to correct the condition. Use Labor Operation N9995.
Service Information: .
Note: The system defaults to Gauge Goes to E when there is a problem. There should be a code stored when the gauge is reading only E. Also note: Currently no secondary senders (left side units) have been found to have a problem. And no sender cards (primary and secondary) have been found to have a problem.
Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed. If these steps do not resolve the condition, please contact GM TAC for further diagnostic assistance. This diagnostic approach was developed for the vehicle with the VIN you entered and should not be automatically be used for other vehicles with similar symptoms.