power supply available?
LED #1 won't light.
do you have a power supply for this iMac?
Depending on which processor you have, it will vary, if you have a 1.6 or 1.8 ghz processor, then you need this one, if you have a 2ghz processor, then you will need this one. However, if you have a 1.8ghz processor, but need EMC no. 2055, then use the one i just suggested for the 2 ghz.
Basically, just check this handy parts page.