ODD fan keeps running, even after sensor change :(

So a few weeks back HDD on my late 2009 iMac 21.5 died. I did some research online and decided to replace the HDD myself as i dont have applecare. I replaced the broken seagate with a western digital HDD and so had to fix the sensor as well. I took the sensor from my optical drive and put it on my HDD as i didnt want it to crap out. I ordered an optical sensor from online but guess what, my optical drive fan was still spinning full speed. So i ordered another sensor from another website hoping i got a bad sensor first time and i still have the same issue. Ran AHT before my second sensor replacement and i got this error "4SNS/1/40000001:TOOP-1.704", which i think is for a bad sensor. I havent run AHT after second sensor replacement yet. I have done multiple SMC resets to no avail. Any clues what could be wrong? Could it be my logic board? Everything on the iMac is running as expected except for this fan. Any suggestions would be really helpful.
Thanks

 

bump!!
No one has any idea what could be wrong?

 

thanks for the suggestion sim, i updated title. Hopefully someone gives me some suggestion, as i am out of ideas now