Gas gauge troubleshooting...

[Aultro]

It's not working and is always on empty and never moves. I've tried two things at separate times.
1. disconnected the brown wire at the tank and and turned the key. The gauge went clear past full and stopped. Went back to empty when reconnected.

2. ran a ground wire to a stud on the back of the gauge and the other end to ground. nothing changed when I turned the key.

Anything else to try? Is my next option to pull the sending unit?

I've had the truck for about 3 months.

Thanks!