iMac Fans running full blast

Since the issue you experienced occurred right after having the system serviced I really think you should take it back to the service provider who did the work. The error indicates a sensor issue related to the hard drive. It may be a simple case of poor workmanship where they left something disconnected. If your system is still under warranty why risk voiding your warranty by opening your own system? I suggest finding another service provider if you're that unhappy with the one who did the work.

The answer to your question depends on which model iMac you have. You indicated you purchased it from Apple with the SSD drive option, so that limits it to the last two 27-inch models. There was some discussion on the boards about changes made to the hard drive sensor between these two models.

http://blog.macsales.com/10146-apple-further-restricts-upgrade-options-on-new-im acs

If I were having problems with a service provider who serviced my product under warranty I would either work with them or give Apple a call to intervene and get the issue resolved. I spent too much money on my iMac to jeopardize voiding the warranty.

If you have the Mid 2011 model iMac as you indicated then there is no way for it to be out of warranty. The Mid 2011 iMac was introduced in May 2011, which means that all of the Mid 2011 iMac's shipped so far are still under the 1 year hardware warranty. In fact, if you are under the 1 year warranty you can still buy the AppleCare Protection Plan if you want to.

If you have the older Mid 2010 model then your system probably is out of warranty.

Since you've been inside your iMac did you happen to notice whether there is a tiny two wire cable plugged into the connector end or the hard drive along with the standard SATA cables? The two pin cable is unique to each brand of hard drive, and must be properly plugged into the hard drive and the logic board. You can see a sample picture of the temp sensor cable at the URL below. If the cable is disconnected, damaged, or incorrectly connected then you will experience the fan ramp issue.

http://www.powerbookmedic.com/iMac-27-Hard-Drive-Temperature-Sensor-p-21364.html

Since you already have the iMac open please be careful. The power supply retains a charge for some time after it after the iMac has been disconnected from AC. I zapped myself once while experiement with an iMac, and it's not something I want to do ever again.

There may be hope yet. Thanks for including the picture. I can see that you have the sensor cable attached, but I am concerned that it may not be attached to the correct pins.

Each hard drive sensor output connector is different. Based on the picture it's hard to tell if the connector is installed properly. From the shape of the PCB on the hard drive it looks like a WD drive. If that's correct the WD drive should have an 8 pin connector next to the SATA connections similar to the diagram below. The two sensor wires (that are part of the 6 position sensor connector on the cable) should be connected as highlighted in green. The sensor cable, assu when attached to the hard drive will leave the left two pins open.

We're running out of things to look at. The only other thing you can do is to verify that the cable is plugged into the logic board and that the pins and/or connector on the logic board aren't damaged.

If any sensor is disconnected the fan(s) will ramp to protect the system. I made the assumption that your system was fully assembled. If you haven't already done so, reassemble the system and then test again. Your problem may be resolved, although it wouldn't explain why the fans ramped as soon as you brought it back from the service provider unless they had left a connector partially removed.

Solution for me was to have the operating system re-installed with the help of Apple by phone.

Temp sensor for 2011 iMacs...

https://eshop.macsales.com/item/OWC/DIDIMACHDD11/