I bought my truck last year and the gas gauge didn't work. I thought it was the sending unit, so finally got around to changing the whole fuel pump assembly with gas gauge. I plugged the new one in and tried it out and it didn't work. I noticed after I dropped the tank and swapped between the old and new one, the old one went up for a second to a quarter tank and dropped immediately twice when trying the old and new one a few times. The new one does not do anything. I haven't filled it up anymore yet, but any other ideas why it might not work.

Cliff notes: Gas gauge didn't work. Changed fuel pump assembly. Gas gauge still doesn't work.
2006 Chevy Tahoe Z71 with 153k

 

Check the wiring at the tank including the plug for corrosion and loose and frayed wires.