Fan running in full speed
Originally, my Imac had an internal sound problem. When playing music, video or whatever that requires the sound system to work, the internal speaker emitted something like alien talk. The technician came to replace the logic board, but then the fan is running in full speed. Now the old logic board is re-used, the sound problem disappeared but the fan is still running in full speed.
Now the technician need to take back my Imac and I hesitate.
I want to open my Imac, is there anything I can check?
Yes there is...I imagine that you reset the SMC or System Management Controller by Shutting down and unplugging all of the Cables, including the power cord.. you need to wait at least 15 seconds, the reset occur's automatically once the Imac has been disconnected from the Power Source.. if this does not take care of the High Speed Fan Issue It sounds like the Tech did not plug in one of 6 Sensor's or one of them was not properly attached and came loose
All of these will cause the Fan's to run at Full Speed...
If you feel like taking the Cover's and Display off, you could look around to see if one of them came unplugged from the Logicboard.
You could carefully follow the take-apart and nose around a bit.. but be careful and Safe.
Check your iMac's firmware ... I have rebooted and made sure firmware was installed before boot process
sequence. Had a similar issue happen also but once the firmware was installed the problem had stopped.
Do not just think that resetting SMC is a cure all solution, although it is a good starting point.
I would double check your issue by rebooting your iMac in safe (boot) mode .. then you will know for sure.
Instructions on safe boot mode can be found via the net same with install of new firmware. If problems persist .. must be escalated to a problem w/ motherboard and/or replacement.. Call AppleCare if your are still under warranty. Best of Luck