iMac not tuning on after logic board replacement

I replaced the logic board on my iMac due to blown L9000 on my logic board. Now when i connect the power cord fans start running themselves without pressing power button. I can hear the HDD spinning. There is no display. The diagnostic led #1 is on but #2, 3, 4 are not on. On my older board same power supply was able to boot my iMac. What could be causing this? logic board issues or something not attached correctly.