A couple week ago I filled my tank and the fuel gauge went to full, after 100 miles, still pinned at full. On the next start it fell below empty. Occasionally (twice in the last month) it goes back to full, only to fall to empty shortly after.

I had the codes read here's what I got:
P0643 (High voltage output from the sending unit)
P1233 (Fuel Pump Driver Module Offline)
B1201 Fuel gauge system not working

I had the fuel pump/sending unit replaced (as per the tsb). didnt fix the problem.

Next I removed the FPDM, it was slightly corroded but not horrible, and plugged it into one of the trucks at my work. The gauge on the other truck worked fine.

So as far as I can tell it is not the sending unit or the FPDM.

Any help would be appreciated, don't want to stress anymore.

Thanks