When I replaced the HDD on my I-Mac, Intel Core i3 running at 3.06 Ghz, there was a small molex type connector on the main board, right near the hard drive, which in my teardown instructions was labelled as 'Internal Temperature Sensor'. They had cautioned me, in the instructions, on removing this connector to gain access to the HDD as the solder connection on the main PCB could crack during removal. Check the solidity of this connector because, if it is broken, it will give a false signal to the Fan Controller circuit. If it has to be re soldered, you have to gain access to the bottom of the board.
There is also a utility that you can download, called SMC FanControl, which allows you to set the cooling fan ON and Off temperature in the i-Mac. I used this utility to maintain my internal temperature to no mare than 120 Degrees Fahrenheit, as the iMac would start to get 'Flaky' at about 140 degrees.
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== Update ==
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Well, as I had used the same instructions when I took apart my i-Mac, I don't remember seeing or reading about any 'prox' sensors. If you are talking about the thermal sensors, these are the units attached to the main board, through the aforementioned cables and connectors, with the fragile 'Molex' connector we have been talking about.
When I replaced the HDD on my I-Mac, Intel Core i3 running at 3.06 Ghz, there was a small molex type connector on the main board, right near the hard drive, which in my teardown instructions was labelled as 'Internal Temperature Sensor'. They had cautioned me, in the instructions, on removing this connector to gain access to the HDD as the solder connection on the main PCB could crack during removal. Check the solidity of this connector because, if it is broken, it will give a false signal to the Fan Controller circuit. If it has to be re soldered, you have to gain access to the bottom of the board.
There is also a utility that you can download, called SMC FanControl, which allows you to set the cooling fan ON and Off temperature in the i-Mac. I used this utility to maintain my internal temperature to no mare than 120 Degrees Fahrenheit, as the iMac would start to get 'Flaky' at about 140 degrees.