RAM will not recognise
I was replacing the keyboard and mouse on a 2009 MacBook Pro that a friend of my families' had. When I had replaced all of the parts it came to the part where'd I have to test my repair job. so I power it on and all, and I'm greeted with a single tone that repeats every 5 seconds, according to Apple, that means there is no detectable RAM, even though I replaced the RAM with the original set of DIMMs. I then tried another matched pair from a 2010 MacBook Pro, and then I'm greeted with three beeps repeated every 5 seconds. Apple listed that as improper or unusable DIMMS are inserted. So the machine recognizes some DIMMs, but not others. And even so it refuses to boot.