What does this sensor code mean...4SNS/1/C0000008 TL0P==124
Replaced my failed HD and now the CPU AND ODD fans are running fast. imac 21.5 mid2010
There is a sensor cable on some iMac models that must be attached to the replacement HDD for the fans to operate normally. I do not recall if the particular models include your mid-2010 iMac but the symptom you describe would be consistent with this requirement.
If you did not buy it from OWC / MacSales or other well-known Apple supporter, it is unlikely to have the required hardware.
Check with OWC to determine the correct replacement you require.