Model A1418 / Late 2012 / 2.7 & 2.9 GHz Core i5 or 3.1 GHz Core i7 Processor

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Sensor Error After Hard Disk Replacement

After replacing the HD I receive this Alert from the Apple Hardware Test:

4SNS/ 1 / 40000000: TH00 - 9.000

Is this the thermal sensor of the hard disk ?

What I've missed ?


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Here's a good reference on the AHT error codes: How to invoke and interpret the Apple hardware tests. While a bit dated it still works for most error codes.


I have the same problem, 4sns TH sensor (Hard Drive Thermal Sensor), the fans running in 6000rpm and mac run very very slow.

Did you find one way to solve this problem?


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4SNS/ 1 / 40000000: T H 00 - 9.000

4SNS - System sensor

T - Temperature

H - Hard disk

As you can see yes the sensor within the HD is not active.

Did you replace the HD with a SSD or a HD? If not your system should still be covered under extended warranty. You'll need to talk with Apple.

If you swapped out your drive then the drive you put in does not comply with the newest SMART spec.

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I replaced a broken HD with a similar model (samsung momentus 1TB).

The Imac is no more under warranty support.

Curiosity: after replacing the HD, I needed to open again the iMac and replace the RAM also, the installation of the OS never went to the end.

After HD and RAM replacing the iMac worked normally.


I think that all new HD are currently supporting SMART


It's the version of SMART thats the issue here. Yes, all SATA drives offer some version of SMART services but over time it's been updated. About two years ago it was updated with a better internal temperature monitoring method which Apple championed.


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Get Macs Fan Control, it can be configured to use other thermal sensors to spin down the HD fan

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@giuliano - Giuliano, overtime we've learned using software is not that smart. The better direction is finding the root cause.


@danj software is all that runs on a computer: it is not an electromechanical mechanism that changes the fans' speed... if Apple software cannot talk to modern drives, what's the problem in using software that does?


@giuliano - Fan control software alters the systems SMC services which is what we don't want as over time it will cause a major failing of the system. The root issue here is the failure of the one of the SMC thermal sensors which needs to be fixed. Unlike the 27" model which uses a 3.5" drive the 21.5" model uses a 2.5" which uses a standard HD unlike the larger system which requires either the correct Apple HD or you need to get a special adapter made by OWC.


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This is a clear and 100% effective solution for all iMacs with 4SNS/1/40000000:TH00-9.000 problem after replacing a HDD:

Please note that Macs Fan Control DOESN'T solve the problem because you still have an error in Apple Hardware Test! Macs Fan Control does only control fan speeds and has nothing to do with a HDD temperature sensor detection.

My solution resolves all the issues after HDD replacing, like:

- Mac OS runs very slow,

- fans are running at the maximum speed,

- kernel task is equal or above 100% in Activity Monitor,

- AHT and ASD are reporting TH00 temperature sensor error.

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I got this same error but did not replace the HD.

Mac won't start. It began showing the flashing folder with a question mark. I looked this up, pressed "R" to do a reset. It started and failed to do the reset. I tried again and again and still showed the folder.

It did respond when I pressed "D" to do a test. Hence this exact error. Would this mean the HD would have to be replaced?

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