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Model A1311 / Mid 2011 / 2.5 & 2.7 GHz Core i5 or 2.8 GHz Core i7 Processor

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Fans running loud - error code 4SNS/1/C0000008 TSOp —124


My imac has the fans running at full speed and Apple diagnostics give me the error:

4SNS/1/C0000008 TSOp —124.

I have used the website below to try to decipher the code, but I haven’t been completely successful.

The codes on the link above, seem to apply to macbooks, not iMacs, since there is no upper-case “S” (only lower-case) and the lowercase “s” refers to the trackpad.

So, this is what I’ve decoded.

4SNS - Sensor, T - Temperature, S-? (capital S), O-Optical drive, p-?

What’s the “C“ in C0000008, is it just an hex address of some sort, or does it mean CPU?

I’ve bought the software TG-PRO, and it shows both optical and CPU fans at max speed and points problems with sensors, but cannot identify which ones.

Should I assume that I have to replace both sensors?

Thanks in advance.

Best regards,


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The error codes in the TechRepublic article lists don’t apply here. Here’s a snippet of the iMac error codes

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And here’s the cable & sensor in question

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Hi Dan,

Thanks for that.

So, both cpu and optical drive fans go like crazy because of the skin sensor?

Best regards,



When SMC looses connection to any of its sensors it assumes the worse! So it goes into CPU Safe mode, ramping up the fan's as well as lower the CPU's clocking.

So focus on the root cause: The cable connection or the bad sensor.


Thanks a million !



Hi Dan,

Thanks a lot. The iMac is now running silent and with no throttling.

Best regards,



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Pedro Cardoso will be eternally grateful.
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