Chosen Solution
got mid 2011 21” imac, i disassembled it all (without removing heatsink), cleaned both fans and motherboard and installed everything back into case. Fans acting as usual on start, but then starts rotating faster, after 1 or 2 minutes. and even more faster and louder if im watching movie. If i close the movie - fans wont get back to normal silent speed( Any guess? Thanks Update (02/24/2019)
sensors
If you haven’t installed the needed thermal sensor you’ll need to do that OWC In-line Digital Thermal Sensor for iMac 2011 Hard Drive Upgrade. Using fanControl software doesn’t really work that well and your system will not last as long. Your system likely needs a bit of dusk cleaning after we see the TG Pro snapshot we can get into wants needed. Update (02/24/2019) I don’t see any issues with the thermal sensors. So SMC is encountering a different issue. I’m suspecting you damaged one of the ferrite beads (L5500 through L5531) near the sensor connectors on the logic board.
Check if all the connectors are connected properly. But before that do a PRAM reset.
