Zestt

Reset to factory settings:
when HP12C is OFF,
1.Press and hold [ON]
2.Press and hold [-]
3 Release [ON]
4 Release [-]
factory set complete

Check all electrical circuitry is working properly (takes like 3 minutes)
HP12C is OFF,
1.Press and hold [ON]
2.Press and hold [+]
3 Release [ON]
4 Release [+]
If all the lCD screen is filled, then 12C is working.