Precision T3600 Error Code Amber 1-2-3 No Video Output

Hi, I am having an issue with a Dell Precision T3600 which has failed to output video. There was initially a motherboard failure [which was tested with a multimeter and proved to be shorted, and the PSU would also shut off upon turning on the system because of the short-circuit-protection]. The motherboard was replaced with an identical one from a working machine but not it turns on with no video output. It was tested with new RAM, a different SSD, no SSD, new CPU and still coming on without video. It had 2 GPUs [tested individually as well] but same issue and I don't think both could have failed simultaneously. Performed a CMOS reset and even replaced the CMOS battery.

Upon booting, it shows a series of codes on the number indicators and the HDD indicator flickers once and then settles on 1-2-3 with the power button on white. Dell's troubleshooting says that 1-2-3 is an error post-video initialization so troubleshoot according to the error on the screen, but I have no display output. Any ideas what can be the issue?