The fans-gone-wild issue and the failed AHT warning is a result of the logic board losing communication with the sensor. Thermal sensors have been included in Macs for as long as I can remember to help the system adjust fan speed to maintain acceptable temperatures. If these sensors do not report any temperature information, the firmware will turn the fans up to full to prevent a core breach in the warp engines, and/or red-hot CPU's flowing like molten slag all over your pretty blue logic board. It's a precautionary measure to prevent an overheating problem. The AHT warning is there to tell you which sensor has failed so you can replace it.
What makes this sensor different is that it is integrated into the custom hard drive. A lot of the earlier thermal sensors were stick-on affairs that attached to the hard drive or other locations with tape. I presume that Apple asked manufacturers to bake them into the drives to reduce both additional component costs and assembly steps.
Wires from the thermal sensor are taped to this hard drive
Unfortunately, this change means repairing a failed hard drive out of warranty (I think hard drives are designed to fail just after AppleCare expires) means paying Apple to do it instead of a cheaper DIY solution. This also means you can't upgrade your drives because Apple will only replace drives with the exact same part. Under their current policies, you simply cannot get a bigger hard drive (or replace it with a SSD) even if you were willing to pay Apple's prices. You better order your new iMac just the way you want it, because it's going to stay that way for a long, long time.
Even more concerning than the design change to force the use of genuine Apple replacement drives is the absolute travesty that Apple has allowed this design flaw of the iMac to continue for so long.
The iMac is the hardest piece of Apple kit to work on yourself on by a long shot. The Mac Pro is designed to be easily opened. MacBooks and MacBook Pros provide easy access to the hard drives and RAM. The new Mac mini has a twist off access panel. Even the old Mac minis are not that bad as long as you have the right putty knife. It's certainly not as bad as the old iBook (about 50 screws to get at the hard drive!), but it's the worst of the current Macs. I don't even think most people need access to the other components inside the machine, but access to the hard drive is important. The old iMacs were designed so that you could remove the back and get quick access to the drives. The aluminum iMacs were designed to open from the front so that you have to remove the glass panel (it pulls off with suction cups) and the LCD screen behind it to get to the drives.
The front glass panel is removed with suction cups
The worst part is that you often put it all back together only to find a piece of hair or lint or a stray thumbprint on the back of the glass. Takes me straight back to the days when I ran a frame shop and would turn a framed photo back over after papering the back only to find a small piece of dust emerge from the dark suede mats, staring at me from under museum glass like a blazing signal fire warning of imminent customer dissatisfaction or framer's rage. That glass on the front of the iMac is likely responsible for more referrals to Apple's generous mental health programs than any other item that has ever sat on a Genius Bar.
It does make for a very pretty iMac though.
Which is, I suppose, the reason why Apple has kept roughly the same iMac design for so long (since 2007). Maybe the drive could be relocated from where it is now, smack dab in the center of the machine, out to the edge where it could be accessed with a removable panel. But you can't add an access panel for the drives to the top or bottom because that has to be kept open for the convection cooling airflow to pull air in at the bottom and vent out the top. So maybe, the side opposite the optical drive. But that would mess up the unibody look of the iMac with no visible seams in the aluminum (just the RAM slot at the bottom). Okay, so maybe adding easy hard drive access is too much of a design sacrifice, but now even if you manage to get at the hard drive, you can't replace it.
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