Unhook the sender unit, and hook up a meter. It should read 0-90 ohms with
a more or less linear swing between full and empty.
If that checks out, then it's a bad connection between the sender and gauge,
or the gauge itself (least likely!).
Not positive but I think the sender and float are one item on the
Tahoe/Suburban. Just had one done on an old 99 Suburban and it cost
over $300 at the dealer. I think the part itself is around $150 retail.
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