Macbook Pro Late 2011 Grey screen

Hi pals,

I have a Macbook Pro Late 2011 since 2012. I've upgraded it to Yosemite, thing that I've to do from scratch because it worked very slow, but that's not the deal here.

The other day I started it, and it simply didn't do it. I've tried everything, like Unique user mode, safe mode, starting from dvd, reseting PMU-PRAM-NVRAM, changing hard disk, changing RAM,... everything I've found in the internet.

Only one thing worked once, and I say once, because it only worked once... I switch the HD and I used an USB to install Yosemite from scratch. It worked but when I reseted it, it never came alive again... That time, I used Disk Utility on Yosemite to format the original hard disk (700GB 7200rpm), and I recovered my personal data.

Now, when I try to start it from USB to install or from HD it never works. It shows the progress bar till 1/3 and then the screen becomes grey, the CPU heats and the fans start to spin like crazy. Sometimes it turns off, sometimes it doesn't and I have to do it.

Any help will be welcome, thank you 😊.