I took a chance on this and it let me down.  This was to replace a 2009 imac that was trouble-free for all these years.  The refurb iMac boots every 1 out of 5 times.  I've tried re-installing the OS, repairing the boot disk, but nope, still trash.  When it does boot up, it runs about as fast as my 2009 iMac.  Should have just sucked it up and "buy once, cry once" from Apple directly.