"Apple used a proprietary heat sensor on the hard drive of this machine. Use an OWC In-line Digital Thermal Sensor for iMac 2009-2010 Hard Drive Upgrade to cure this problem
OWC In-line Digital Thermal Sensor for iMac Late 2009 - Mid 2010 HDD or SSD Drive Upgrade"
I would check to see if you lost a resistor in the process. Seems common that if you have a CPU fan which reads 0 rpm but seems to be going full blast, there's likely an issue with the circuitry on the logic board. What a few people have found in another thread is that there's a 47KΩ resistor which is missing near the power button connector. If you have one missing from there, all you need is a new resistor and your Mac will function CORRECTLY because it knows the fan speed. Link: CPU Fan runs at full speed.
"You're right you have a sensor issue. The sensor reports to the EFI that it can't detect/control the FAN so the EFI runs the fan at full throttle (by default in scenarios like this) to keep you from cooking your CPU.
I don't know if you damaged the connector, blew up the sensor but the error is basically saying it believes your FAN MOTOR (4MOT) for the HD (HDD) is out of spec. and the amount it's wrong is the last set of numbers in RPM (I suspect too slow - hence kicking it into overdrive).
MAYBE you need an EFI update to run that drive? What is it's SATA Speed (your bus is 3Gb/s Max)
Have you tried an SMC reset?
If this answer is acceptable please remember to return and mark it."
I'm having the same issue, although its not immediate and istat shows ~1400 RPM before the issue occurs. then when it occurs, the fan goes to full speed and shows 0 RPM. i am not knocking my iMac at all. it just seems to happen on its own during use
