iMac: Fans run at full speed after computer turns on
Symptoms
After plugging in the iMac and turning it on, the fans (blowers) may be noisy and appear to run at full speed. In this event, shutting the iMac off and turning it on again wouldn't fix the issue.
Products Affected
iMac (20-inch, Mid 2007), iMac (24-inch Mid 2007), iMac (Early 2006 20-inch), iMac (Early 2006 17-inch), iMac (24-inch, Late 2006), iMac (20-inch Late 2006), iMac (17-inch, Late 2006 CD), iMac (17-inch, Late 2006), iMac (20-inch, Mid 2009), iMac (21.5-inch, Late 2009), iMac (27-inch, Late 2009), iMac (24-inch, Early 2009), iMac (20-inch, Early 2009), iMac (20-inch, Early 2008)
Resolution
If you press the iMac power button while you are inserting the power cord, the iMac will enter a mode in which the fans (blowers) run at full speed. In order to correct the issue, reset the SMC and then start the computer by pressing the power button after the power cord has been fully inserted.
The iMac should now start up and operate as expected.