Someone put in a replacement HD and forgot to add in the needed hardware sensor if they put in a non-Apple drive the system came with. Here is the needed sensor: [https://eshop.macsales.com/item/OWC/DIDIMACHDD11/|OWC In-line Digital Thermal Sensor for iMac 2011 Hard Drive Upgrade] and here is the needed steps to get to it: [guide|5954|iMac Intel 21.5" EMC 2428 Hard Drive Replacement]. You can stop as Step12 to slide on the sensor for the drive you have now.