I have replaced the HDD in my 2011 27" iMac (10.11.6) with an SSHD. The fan is now running at full speed due to the new drive not having the relevant sensor information. I have fitted one of these 3rd party sensors but the noise persists. I have tried software solutions such as SSD Fan Control and HDD Fan Control, thay made no difference. I have reset SMC many times, as well as NVRAM. Does anyone have any other ideas please? Apple hardware test returns error code 4mot/4/40000003:hdd-1244 which refers to the HHD temp sensor I believe.