problem with replacing logic board

I have an 20‘‘ iMac 4,1 with a 2 GHz Intel Core Duo (modell 1174, logic board 820-1888-A). Installed is Mac OS X 10.6.8. Because I wanted to install some applications which need a OS X version greater than 10.6.8 I bought a used logic board 820-2031-A with an Intel Core 2 Duo. I used the iFixit-guide to replace the logic board and until then everything went find. But after finishing my work the iMac didn‘t power on. Cause I were not sure, if I hadn‘t made a mistake, I repaced the old logic board. The result was, that the iMac started normally without any problem. I mention, that the problem is something with the C2D-board. I found some threads about iMacs not powering on after logicboard replacement and I started to figure out what the problem could be. I tried to reset the SMC and put a new battery in but without any result.

So I tried to find the diagnostic LED‘s of the board, but on the board 820-2031-A I can only find 3 diagnostic LED near the bottom and not the described 4 LED‘s. Is this ok or is there somewhere another, the 4. LED? If it is correct, that the board has only 3 LED‘s, what are the functions of those? Are they simular to the first 3 LED‘s of the discribed 4 LED version?

And the last question: Is it possible to test the board outside of the iMac? If yes, what is the way to do this?

I hope you can interpret my bad english an anybody can give me some hints. Thanks from Germany.