2011 iMac CPU fan running full speed

[ElCani]

I have a 2011 27 iMac (2.7, 12GB, 2TB HDD + 250GD SSD Fusion drive). A week ago the PSU failed, which I replaced yesterday. Everything was fine for over 24 hours until suddenly in the middle of me using the iMac, the CPU fan suddenly ramped up to full-speed - and stayed there. The fan speeds up at the login screen phase of the boot process. I've tried SMC resets and NVRAM resets with no luck.

I've got the fan under control using "Macs Fan Control" but obviously I would like to know what's wrong. I've installed iStat Menus and can't see anything obviously amiss with the readings. I've attached a screenshot when the fan control software is disabled and the CPU fan is speeding away.

Could this be related to the PSU replacement? To reiterate, everything ran perfectly for over 24 hours after I fitted the new PSU. Activity Monitor shows no weird processes using lots of CPU.

The HDD is the original (I know that replacing these with non 'official' drives causes the fans to run at full speed).

Any ideas or advice would be much appreciated.

 

[briloronmacrumo]

the CPU fan suddenly ramped up to full-speed - and stayed there.

The HDD is the original (I know that replacing these with non 'official' drives causes the fans to run at full speed).

I would:
(1) run Apple's Hardware Test https://support.apple.com/en-us/HT201257
(2) boot into single-user mode ( boot and hold CMD+S ) and run 'fsck -fy' on the hard drive.

The issue with the "official" HDD is a sensor cable on the 2011 iMac connected to the HDD. OWC has a kit that allows a non-official HDD to be installed. Just do a web search for OWC, 2011 iMac and fans and you should find all the appropriate hits. Here is the OWC HDD kit http://eshop.macsales.com/item/OWC/DIYIMACHDD11/

Maybe the current sensor was knocked loose during the PSU replacement?

btw: Regardless of what tests show, I would replace(preferably with an SSD) a 5-year old HDD inside an iMac while I was inside it anyway. Many HDDs fail at this age.

 

[ElCani]

Thanks for the reply.

I ran AHT and got the following error code: 4SNS/1/40000001:ID2S-0.00

Googling that didn't come up with anything particularly useful. I followed the advice about booting into Single User mode and ran fsck - fy, which showed no problems.

Re the HDD, I don't think the sensor could have got damaged during the PSU install as I didn't touch it. Perhaps it's the LCD temp sensor or cable which was disconnected/connected during the process? I think I'll take it apart again and check that everything is seated/connected properly.

I can't really afford a 2TB SSD to replace the current HDD. I have a 250GB SSD under the Superdrive and I'm quite happy with the performance of the Fusion drive set-up. Obviously if it is the temp sensor in the HDD that's causing the problem that I will replace it, but it seems like it could be one of a number of different sensors that is to blame.

 

[ElCani]

So... taking it apart and putting it back together didn't work. Also, I just ran a Geekbench 3 test and it scored roughly half what it should do :-/. I ran this yesterday and it was normal. Some kind of throttling?

 

[Dopeyman]

Try resetting the SMC and PRAM as shown here...

 

[briloronmacrumo]

I ran AHT and got the following error code: 4SNS/1/40000001:ID2S-0.00

Any error code showing up is bad news. AFAIK, only Apple knows what those codes mean. Do you have an Apple store nearby? One of their geniuses should be able to look it up
[doublepost=1454131219][/doublepost]

...Also, I just ran a Geekbench 3 test and it scored roughly half what it should do :-/. I ran this yesterday and it was normal. Some kind of throttlling

Throttling usually indicates the machine thinks it is running too hot. IIRC a bad ( or missing, or not completely attached ) sensor cable will cause this behavior and thus the throttling. If you really want to keep a five(5) year old hard drive, it should be possible to simply use OWC's sensor cable. I'd find out what the AHT code means first before spending any more time.
[doublepost=1454131317][/doublepost]

Try resetting the SMC and PRAM as shown here...

The OP's first post indicates he's tried this.
[doublepost=1454131788][/doublepost]

I ran AHT and got the following error code: 4SNS/1/40000001:ID2S-0.00

Googling that didn't come up with anything particularly useful.

Did you see this article? Got it on my first google: http://www.cnet.com/news/how-to-invoke-and-interpret-the-apple-hardware-tests/
Seems to say its definitely a system sensor failure.

 

[ElCani]

Did you see this article? Got it on my first google: http://www.cnet.com/news/how-to-invoke-and-interpret-the-apple-hardware-tests/
Seems to say its definitely a system sensor failure.

Cheers. That seems to suggest it's a DC current sensor, which fits with the power supply issues. Perhaps the replacement PSU (which was a used part) is faulty? I think I'll return it and try another one.

 

[ElCani]

Just to update this, the fault was with the replacement PSU. I've fitted a different one and the iMac is running fine again (normal Geekbench, AHT shows no problems). So, it appears that the AHT error code "4SNS/1/40000001:ID2S-0.00" relates to a sensor on the PSU.

Thanks to all who replied.
[doublepost=1455038727][/doublepost]Another quick addition. iStat Menus now shows results for three "Power" sensors that were not visible when the problem was occurring. Compare the screenshot below with the one I posted above.