There are diagnostic lights on the logic board please take a look at them and tell us what they are showing.
Location Name Color Status Indicates
LED 1 Yellow On Yes Power supply is providing trickle power
LED 2 Red Off No CPU B halts on an IERR (instruction error)
LED 3 Red Off No CPU A halts on an IERR (instruction error)
LED 4 Red Off No CPU B is over temperature
LED 5 Red Off No CPU A is over temperature
LED 6 Green On Yes EFI has configured the graphics card
LED 7 Power on Green On Yes All power rails are functioning
LED 8 EFI done Green On Yes EFI is done loading GPU present
CPU A (lower
processor)
Overtemp
CPU B (lower
processor)
Overtemp
CPU A (upper
processor)
Error
CPU B (lower
processor)
Error
Trickle
power
LED Button
to Display
436 Troubleshooting a Mac Pro