oil pressure gauge shows 80 psi at all times, but returns to zero when key is off

Question

2003 GMC Denali fully loaded AWD runs fine one day the oil pressure gauge worked fine the next day when you start up the pressure gauge shows max (80psi) doesn't move at all except when you turn the key off & shut down engine, start the engine and it maxes out and stays there, no valve noise, so I know the motor is pumping oil, think it could be the oil pressure sending unit, what is your thoughts ?

Answer 1

Its the dash GMC, and Chevy are plaque with these failure