According to [http://support.apple.com/kb/HT2538|Apple] this means that the RAM does not pass the test. There is a RAM DIMM in your computer that is defective. Please follow [guide|990|these] directions to take out your ram. Start by taking out one, then starting up the computer. If it works, that RAM DIMM (or RAM stick) is defective. If it doesn't work then its the other DIMM, so reinsert the one you just took out, take out the other, and start up your computer again. In the unlikely event that it still doesn't work, it may mean that both DIMMs are fried and you need to order [http://www.ifixit.com/Guide/search/200+pin+6400+RAM+chip|some ram] and test it out with that. Your iMac has a maximum ram capacity of 6 GB.