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Model A1225 / Mid 2007 and Early 2008 / 2.4, 2.8, or 3.06 GHz Core 2 Duo processor

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PSU Overheating? Loud Fans - replacement PSU?


I can see that this machine and fans have been discussed many times on this forum I've looked through the Questions here on iFixit and elsewhere but can't find a definitive answer to my problem.

I have a 24-inch 2.8GHz C2D iMac Early 2008 Model.

This machine has been wonderful since I've had it and I love it. About two months ago I noticed the Fans starting to get quicker and louder. Nothing in the iMac had changed prior to this occurring.

I have ran AHT on loop and no problems found.

I have dismantled the iMac to clean out all the fans and air vents.

Its not always the same fan that is going full pelt and sometimes its more than one.

Temperatures are as follows (give or take a few 'C)

HD - 51c

CPU - 45c

CPU Heatsink - 47c

Airport Card - 42c

Ambient - 30c

GPU - 60c

GPU Diode - 62c

GPU Heatsink - 58c

PSU - 90c

From what I have read with the exception of the PSU the above temperatures seem normal. I'm guessing that as the PSU doesn't have its own Fan that the other fans are kicking in to prevent damage.

If I reach around the back of the iMac from the front the middle/top/left of the case is extremely hot and can only be touched for a few moments before you have to remove your hand.

Occasionally after long periods of work the iMac switches off and I have to restart.

I have reset SMC, PMU done AHT etc...

Can anyone advise or share their experience if they think that a replacement PSU would solve the problem or is it possible that this is just masking a deeper problem with my iMac.

Many thanks



Sorry for the delay in responding. I've been away doing other stuff.

No i've not resolved the problem yet - i've been living with it and have suffered the odd cut out due to over heating.

Just to recap - the replacement power supply didn't resolve the issue.

I suspect its the graphics card or logic board so will probably live with it for a bit and then sell it for spares/repairs.

If anyone has any input will be much appreciated.



Sorry for the delay in updating this post. I have only just been able to get my hands back on the iMac for further testing.

I replaced the PSU but unfortunately it has not resolved the issue and after about 10 mins of use the fans were cranked right up again.

I noted the temperatures in my original post of the components and here they are again now as I am using the iMac.

all in Celsius

Ambient 27

CPU A Heatsink 44

CPU A Proximity 45

CPU A Temp Diode 42

CPU Core 1 45


Display Screen Prox 51

Graphics Processor Chip 1 56

Graphics Processor Heatsink 1 56

Graphics Processor Tem D 58

Hard Drive Bay 1 50

Main Logic Board 47

Optical Drive 41

Power Supply Position 1 84

Wireless Module 55


Optical 699rpm

HD 1200rpm

CPU 3086rpm

From what I have read all the temperatures seem to be normal and the PSU temp is the only one that is causing me some concern. I'm not sure whether it is perhaps a faulty sensor or sensor cable and whether anyone has experience on this?

I am 100% confident that it is not software. I have another identical iMac here that I have loaned to my customer and 'cloned' the hard disks and the problem has stayed with the iMac rather than the software. At the moment I only have Text Edit, Safari and Activity monitor open and the system is 93% Idle.



Recent update.

I left the iMac running AHT on loop over night and it through up an error message 4SNS/1/40000000:Tp0P-93.050

Apple Service Source response to this error code

Suspected part - Power supply sensor pin/power supply,

Suggested Action - Verify that the single pin connector on the top edge of the power supply is present and securely plugged in. If connector is damaged replace the DC Power Supply cable. Notes - If this sensor signal wire is disconnected or faulty the CPU fan will run at full speed.

I have tried a replacement Power Supply that did not solve the problem, the sensor pin on both Power Supplies looked fine. The pin connector is securely plugged in.

The problem is that the Fans run at full speed all the time only after a few hours of use which is probably why the AHT got this message only after being on test for 2.5 hours after it had chance for the Fans to get up to speed.

I may try getting a replacement DC Power Supply Cable but I'm not convinced and its a !@#$ of a job!

Отвечено! Посмотреть ответ У меня та же проблема

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Hi Simon,

di dyou finally managed to solve this problem? Mine presents the same problem



Carlosnur - Did you install any temp & fan monitoring app to see what is going on? I would start there. In the meantime how about creating your own question so your problem doesn't get confused with Simon's.


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There are many causes for heat type of issues:

Your application work load is high given the system resources - Needs more RAM and/or HD space. Start by cleaning out old stuff and test & defrag the HD. Monitor RAM & HD usage & space.

You have malware or other unwanted software running on your system. Install a good antivirus program & monitor CPU usage using Activity Monitor Heres a good story on what you can get onto your system without trying Download me II—Removing the remnants of the Web’s most dangerous search terms.

You've already cleaned out the dust which is a very good start. And yes, Your PSU could be having a problem. But you could also have a bad fan or as I pointed to above a software level issue.

I would run a better application here to monitor things than AHT. Download from the App Store Temperature Gauge or get ahold of some other good temp & fan monitoring app. You do want one that can record things so you can review it afterwards. Use your system as you normally do and see if you see something over a period of time. Do you see a pattern? that maybe what you need to isolate out what is going on here.

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Thanks for your input.

When the problem first started to happen it was running Leopard and had 2Gb RAM it has since been upgraded to Lion and now has 4Gb RAM.

The fans start to activate a few minutes after the machine has been switched on even if it has been left idle with nothing running and the activity isn't showing any unexpected process or one that is taking a lot of resources.

As it isn't always the same fan I don't think its a fan issue and if there is a component in the Mac which is 90c + and its v. hot to touch I think having the fan spinning is a good thing!

So basically the only pattern is that the PSU is hot after only light/no usage?

I am kind of convinced that it must be the PSU but as I haven't read this same problem anywhere else I'm kind of sceptical as normally you would expect someone with the same solution.




Did you install Temperature Gauge? it can help to isolate out if the PSU is running hot. True, running fans is a good thing! But they shouldn't need to be running with the system idling.


Is this the original HD or a replacement? If it was replaced what make and model was put in?

At this point I would try running Activity Monitor to see what processes are consuming the CPU. While this may appear far afield please do it.

I want you to see the what process are running here at the top as I think I know whats happing here. And, no you don't have a software problem.


No its the original Hard Disk. Normally the Activity monitor is showing 93% idle but one time there was a Flash process that was taking about 60% when it kicked in.


OK - Now you've got me scratching my head ;-{ I was expecting the indexer process running.


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Possible fault with the motherboard? I am going through a 2008 24" iMac with a very hot power supply as well. Hm!

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I had a Mid-2010 Core i3 iMac, which had a sticky fan (100% for ~24hrs), It happened three times, about a month apart, and when I finally ran AHT Diagnostics, it gave me an error code that said the entire graphics subassembly was bad.

The local Mac shop could not help me, even with the code (as they were trying to replicate the problem, and had only had the computer for 2 weeks). I went over their heads, as I had about 1 month of AppleCare left, and had it replaced with an exemption code.

I have since updated to a Late 2013 iMac, but wish I still had the (upgradeable) and super-quiet original...

The Late-2013 iMac has serious issues with my 2TB Seagate external for backup purposes... Ejecting it without any reason or warning (dang USB3.0 ports!!)

Don't ignore the graphics subassembly as the culprit tho... My temps were ridiculous for both PSU values, but it was graphics that was the culprit...

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Hello Simon, have you gotten an answer to fix your issue?

My friend gave me his iMac whose HHD failed. I installed a new one and installed the OS and all, and I see temperatures of 70 and up from the Power Supply. I noticed the heating begins exactly where the power supply is located. I also noticed the fans and heatsink are positioned below the power supply. Blowing air through and cooling it that way. I wonder if the issue is with the cooling system and not the power supply. Usually a power supply if it malfunctions, it just stops providing power. If the power supply is not being cooled properly, it will heat up continuously. To test the cooling sink I am afraid I will need to take the entire thing apart. I've been trying to avoid this, and searching to see if someone has found a different solution.

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