Mid 2010 21.5" iMac will not power on.

When I press the power button on my iMac you hear the fan start up and stopping. The boot up cycle will not continue. I can leave my mac on and maybe the cycle will continue after 5, 10 minutes or not until hours later or not at all.

Now, I have done all the resets that one can do (SMC etc) to no avail. I have opened up my iMac, checked all the wires, changed the battery on the back of the logic board but also didn't solve my problem.

With the mac open, and the working power supply plugged in in a working outlet the Led #1 comes on.

Then, when I press the power button: Led #2 comes on, the fan starts and led #2 goes off again, stopping the fan. And it goes on again, the fan starts to rotate and the led goes out again and the fan stops. And so on etc... The boot up sequence continue. It gets stuck in some sort of loop.

So, before I spend hundreds of euro's on a new logic board, I really want to rule out any other problems with ie. the power button or anything.

If anyone can give me a straight answer, it would be much appreciated....