Replaced ATI Radeon HD 4670 256 MB. Machine powers on, but there is no response on the display. Board gives first 3 diagnostic LEDs, but does not give the 4th. Was able to do PRAM reset and the machine chimed every time. After waiting a while on the black display the user’s desktop shows up on the screen and I can use the machine with no issues. I can also option boot, but I can’t see what I’m doing since the display is black the entire time until I boot into a drive. I’ve swapped out the display with multiple other good displays and the same issue occurs. Is this a sign of a bad GPU?