Mid 2011 iMac shuts off while loading error4SNS/1/40000000: TH00-9.000

Happy New Years!

Shuts off while loading....maybe 30 seconds after turn on. (Mid-2011 iMac Intel 21.5" EMC 2428)

Ran the Apple Hardware Diagnostic and got error: 4SNS/1/40000000: TH00-9.000.

This error seems to be related to a Temp Sensor on the HD. (Of note, i had to use the Mac Fan Control application recently to turn down the fan as it was running at full tilt).

I was able to run the Disk Utility and tried to repair the disk.....get an error:

Verify and Repair volume "Macintosh HD".

Repairing file system.

Checking Journaled HFS Plus volume.

Checking extents overflow file.

Checking catalog file.

The volume Macintosh HD could not be verified completely.

File system check exit code is 8.

Updating boot support partitions for the volume as required.

Error: Disk Utility can't repair this disk. Back up as many files as possible, reformat the disk, and restore your backed-up files.

Error: Disk Utility can't repair this disk. Back up as many of your files as possible, reformat the disk, and restore your backed-up files.hard drive temperature sensor

As background: This imac was bought broke off ebay and i had a local Mac repair guy replace the main board and install a Terabit HD and the mac worked for a couple years now. Recently, it started running the fan at high speed and so we used the installed Mac Fan application to bring that down to normal.....used some settings, but cant remember what was changed exactly. That worked until now as it wont finish loading.

Do I have a bad hard drive and a bad temperature sensor? If it is a bad temp sensor, is that a logic board replacement (hope not!!).

Appreciate any advice on a next step.