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iMac Fan loud and 4SNS Error in dignostics

My Mid 2011 iMac fan starts to go loud after about 20-30 seconds of being on, and then after a period of time (5-10mins but varies) it shuts down. I am ok with opening up and doing some repairs myself inside the mac. I get the following error in Apple diagnostics.

4SNS/1/40000001: ICOC —114.06

The error is pointing to a System Sensor, Electrical circuit on CPU.

I have replaced the Heat Sink with a new one, including changing the temperature sensor for CPU, this has not fixed the issue.

I have downloaded fan control and limited the fan speed and been keeping an eye on temperatures, nothing really rises above 50 at the time of crash.

Tried PRAM/SMC resets etc, this does not help.

Any help would be really appreciated.

Update (11/30/2019)

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Screenshot of istat menu readings. Interestingly the CPU core reading is not registered. Maybe this points to something?

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Found them on the board, thanks

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Well, you’ve hit an error I haven’t seen! Looking through my notes from all my Apple training I don’t even see a reference to it! So its a rare bird for sure!

But there are some clues here the first is the prefix 4SNS which tells us its a sensor. As you replaced the CPU thermal sensor and the heatsink I’m thinking the sensor is not thermal but voltage! This gets into the the VRM’s and the onboard CPU sensors. Remember not all of the SMC sensors focus on thermals and some are not directly serviceable as well!

At this point I’m leaning on replacing the logic board as the next direction iMac Intel 21.5" EMC 2428 Logic Board Replacement

Update (11/30/2019)

My schematic was damaged so I needed to get an update here’s the sensor!

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Thanks Dan. It is indeed rare as I’ve spent hours trying all sorts in trying to fix this. I really didn’t want to go down the route of replacing the whole logic board, its a spare test machine. Was hoping it was something someone has come across and resolved. All LEDS on the logic board light up which made me think it was not the logic board and nothing came up in the Diagnostics about the logic board either.

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Also, wouldn't ICOC in the error indicate it's an electrical current sensor not Voltage as it would have been had a V in for it to be Voltage?

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@faisalgondal - Well, I found it! I need to get another copy of the schematic to find it. Check my update.

Yes, ICOC would be current not voltage. Often the sensors do dual duty.

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Thanks Dan. I look forward to the update

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Dan - is this something that is replaceable or fixable?

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Faisal Gondal zal eeuwig dankbaar zijn.
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