hardware test 4MOT/4/40000003:HDD-1291 not sure what to fix

Dear everyone,

I think this is my final try to fix my old 2009 iMac.

The iMac fail to boot into the OS and reboot and hang itself. It was already a year.

I believe the main hardwares (monitor, cpu, graphic, ram) they are still in good condition. I was able to run Apple hardware test and got the error code as title. The end of the error code wasn't fixed, means sometime it will change but always look similar, 12xx kind of.

I did some google and people were suggesting about fan or heat issue. But I couldn't score the solid answer of how to actual repair (or if it can be repair at all)

I have good experience to change the iMac hard drive and made it dual HDD (ssd + HDD) by myself, but beyond that I never touch the main board. But I am OK to try, because if it cannot be repaired by myself I will trash it.

Thank you any of your input!