No, if it is LED #1 it idicates your power supply is fine at providing start power,,, the other 3 indicate major failuse of different hardware trees, particularly those dealing with video.
• Indicates that the trickle voltage from the power supply has been detected by the main logic board. This LED will remain ON whenever the iMac is connected to a working AC power source. The LED will remain on even when the computer has been shut down or put to sleep. The LED will turn off only if the AC power source is disconnected or the power supply is faulty.
• Indicates that the main logic board has detected proper power from the power supply when the computer is turned on. This LED will be ON when the computer is turned on and the power supply is working correctly.
• Indicates that the computer and the video card are communicating. This LED will be ON when the computer is communicating properly with the video card. If LEDs 1 and 2 are ON and you heard the startup sound, but LED 3 is OFF, then the video card might be installed incorrectly or need replacement.
• Indicates that the computer and the LCD display panel are communicating. This LED will be ON when the computer is turned on and video signal is being generated. If the LED is ON and there is no image on the LCD display panel, the LCD display panel or inverter might be installed incorrectly or need replacement. 5. LED #4 should be On when the video card and LCD display have communicated properly to produce a video image. If this LED is off you can verify the VGU is working by connecting an external monitor… if you see no image it points to a problem with the VGU - if the image is normal it indicates an internal video chain fault.
Only you know if you damaged connectors at disassembly. Only you know if you reassembled everything correctly (by that I mean connectors making full and complete contact). (ribbon cables inserted right side up and all the way, cams locked down, connectors snapped in straight and completely) If you are fortunate you only need to complete some connections… if not you may need a new logic board or repair of the old board.
If this answer is acceptable please remember to return and mark it.