Apple iMac (Early 2006) - Manual (Page 78)

78 Chapter 4 Installing Memory
If your iMac doesn’t recognize the memory or it doesn’t start up correctly, shut it down
and check the installation instructions again to make sure that the memory is
compatible with your iMac and that it is installed correctly. If you still have problems,
remove the memory and consult the support information that came with the memory
or contact the vendor who provided the memory.