Hey guys, we are fixing up an iMac at my store and this one has really twisted my thought process and I have no idea what steps to take next.

iMac logic board was replaced. iMac doesnít have screen assembly (missing LCD and glass).
When started the iMac loads about 3/4 and boots to a white screen and hangs there.
iMac however DOES boot into safe mode (slight lines when entering user) and works flawlessly in safe mode (on external display)
Steps Iíve done:

Reset NVRAM/PRAM/SMC several times

Reinstalled the OS (Capitan) on several drives
Verbose mode, no errors
Single user, checked file system no errors
Ran driver cleaners/AV, no problems
AHT boot gave no errors ...
OS receives all updates in safe boot, still no luck

Next step is to try and replace the actual SMC battery ..
Iím just wondering if the Mac is booting but not using the external display as the main one (hence white image) .. at this point Iím extremely clueless and need help.