No boot, no chime, LEDs #1 and #2 lit, #3 not lit

This iMac went through something catastrophic, and initially was completely dead, nothing when pressing the power button, no LEDs lit on the logic board. Replaced the power supply, and then LED # 1 lit up. Replaced the logic board with a used one from ebay from a vendor I trust who said it was pulled from a working system. Now, LEDs #1 and #2 are lit, all the fans come on, and the white power LED is also on. LED # 3 shows nothing and no video or POST chime. Thinking it was just bad luck, I talked the vendor into sending another logic board from a working system. Same result. If the GPU on the logic board is not the cause of #3 not lighting, what else could it be? By the way, I replaced the original board with EEE code XYE with ones with XYD and WZH.