Hi all, I need some help. Please pardon my poor English. I just added a Crucial SSD in a 21.5" 2011 iMac, using iFixit tutorial. All went fine, no issue at all. First boot after re-assembling : the CPU and optical drive fans run at full speed. The hard drive fan is ok. An SMC reset did not solve the problem. I installed MacsFanControl on the HD to slow down the fans, MFC offers to use the CPUs or CPU heatsink thermal sensors and the optical drive sensor, so it seems that MFC "sees" the sensors. Then, I installed El Cap on the SSD and used Migration Assistant to migrate the accounts from the HD to the SSD. First boot from the SSD : the fans are silent, MFC is not needed, all run smooth as expected. Having some maintenance to do on the HD, I boot on the HD, then reboot in safe mode. As a result, the CPU and optical drive fans run at full speed again. SMC reset does not solve the problem. (NVRAM reset just in case). I open the Mac again, check the connectors, unplug and plug back the thermal sensors connectors, complete the re-assembling and boot from the SSD : fans still running at full speed. Just in case, I reinstalled El Cap on the SSD without erasing it. The problem is still here. Now I have no idea about what to do to solve this issue. - MacsFanControl "sees" the thermal sensors, and when using them, it does regulate the fans speed. - iStat Pro does NOT see the sensors, it only displays : HD temp SSD temp Ambient temp No CPU, no CPU heatsink, no GPU, no optical drive temp displayed. What do you think : hardware or software issue ? Did I hurt the connectors in any way, despite they look fine ? Thank you for any help in advance.