Late 2009 iMac fan always full speed

Reset the SMC.

Have you tried an SMC reset yet?
Intel-based Macs: Resetting the System Management Controller (SMC)

It could mean that optical drive thermal sensor gone bad. Run AHT in Extended mode. If I am right, you should get an error code related to this part.

deafgoose said:

I can't run AHT.

I have a late model 2009 iMac running OS X Lion.

Apple discontinued AHT.

Click to expand...

You could try this?

And if that doesn't work, you probably still have your Mac OS X 10.6 Snow Leopard DVDs around to run AHT.

deafgoose said:

Could I just purchase the sensor and replace it myself?

Click to expand...

If you feel comfortable to open up your iMac, then yes, it's doable. You need to remove glass and LCD. Sensor is bundled with cable AFAIK (ask seller before buying to be sure), for 21.5" you need part number 922-9214, for 27" it would be 922-9229. It costs few dozen bucks.

I'm afraid that no software is able to control this fan when its sensor gives wrong info to the logic board (i.e. fan is working in kinda "emergency mode").

You can order part here: http://www.macpartsonline.com/imac-parts.html and replace it yourself if you feel skilled enough.

Sensor is bundled with cable AFAIK. At least service manual lists it as one part number. You got the number (922-9214) so ask seller if it's as I wrote.
http://www.macpartsonline.com/922-9...5-inch-3-06-3-33ghz-imac-late-2009-a1311.html

deafgoose said:

Thanks sheep! You have been so helpful to solve my problem. I really appreciate it.

I sent them an email asking if the temp sensor is included with the cable.

Click to expand...

Sorry to resurrect an old thread, but what ever came of this? I had a strange experience last night with the fans going full blast. But mine corrected by putting it to sleep, leaving it that way while I watched a movie and then when I returned and woke it up, it was fine.

GGJstudios said:

Reset the SMC.

Click to expand...

This solved my problem of fans constantly on high in Snow Leopard. Thanks!

There are a LOT of different things that can cause the fans to go full blast, but it ultimately boils down to the system either thinking that something is overheating or the system not knowing the temperature and running the fans to be safe.

From the OP's iStat picture, it was clear that the optical drive temperature sensor was acting up, since it wasn't in the list of temperature sensors. Granted, it could still be the logic board itself not seeing the sensor correctly.

Not, to rekindle an old discussion, but I have a similar problem with Mountain Lion on an Early 2009 iMac.

I have iStat Menus installed and when the fans go berserk, I notice that half of the temp sensors go "missing" (they have a "-" in place of their temp). If I reset the SMC, I have a 4/5 chance that the system will return to normal for a period of time, but the sensors and fans will eventually go haywire even if I don't use the computer much.

I just reset the SMC, so I can't take a screenshot, but I'll add one the next time it happens.

Is there a way to get the hardware test to burn onto a CD? I cant find the original system disk, and I cannot get it see the supposedly online version from the apple support page.

What are people's opinions on tools like TechTool Pro, Drive Genius and the like? What happend to the TechTool Deluxe that used to be available for AppleCare?

- Adrian

Screenshots! (It happened again after 30 min of minimal usage..)

adriandlm said:

Not, to rekindle an old discussion, but I have a similar problem with Mountain Lion on an Early 2009 iMac.

I have iStat Menus installed and when the fans go berserk, I notice that half of the temp sensors go "missing" (they have a "-" in place of their temp). If I reset the SMC, I have a 4/5 chance that the system will return to normal for a period of time, but the sensors and fans will eventually go haywire even if I don't use the computer much.

I just reset the SMC, so I can't take a screenshot, but I'll add one the next time it happens.

Is there a way to get the hardware test to burn onto a CD? I cant find the original system disk, and I cannot get it see the supposedly online version from the apple support page.

What are people's opinions on tools like TechTool Pro, Drive Genius and the like? What happend to the TechTool Deluxe that used to be available for AppleCare?

- Adrian

Click to expand...

Follow-up:
After another SMC reset (I hate working with those loud fans), the problem came back after 30 min, but this time it happened gradually. The CPU Heatsink sensor showed "-" first, and on its own. And only the CPU fan went to 100%. Then after 10-15 min, half of the other sensors went kaput in iStat and all fans went on full drive.

Also, I found my old iMac system disks! The AHT showed no problems on the quick test, and I will run the extended test overnight, as it says it will take a long time.

(If anyone still reads this discussion, I'd love so help!)

- Adrian

Edit: Extended test results (unfortunately, theyre bad)
What should I do?

I don't know what the code means, but you could Google it. No software tool is going to fix your issue, since you have a hardware problem. Tech Tool Pro is a complete waste of money, and Drive Genius is almost a complete waste.

Anyway, the reason your fans are going nuts is specifically because the temperature sensors are going offline. The options are that it's either the SMC going bad or that one (or more) of the temperature sensors are going bad. Yes, despite the fact that multiple sensors are going offline, it's still possible that it's only one. Beyond that, knowing which one is the problem boils down to trial and error. Unfortunately, they intentionally put different connectors on them all so that people can't confuse the system by plugging them into the wrong jack on the logic board, so that also rules out moving them around. Someone is going to have to swap in known-good parts and see what happens.

Running Fans May be faulty 1TB Seagate Hard Drive

adriandlm said:

Follow-up:
After another SMC reset (I hate working with those loud fans), the problem came back after 30 min, but this time it happened gradually. The CPU Heatsink sensor showed "-" first, and on its own. And only the CPU fan went to 100%. Then after 10-15 min, half of the other sensors went kaput in iStat and all fans went on full drive.

Also, I found my old iMac system disks! The AHT showed no problems on the quick test, and I will run the extended test overnight, as it says it will take a long time.

(If anyone still reads this discussion, I'd love so help!)

- Adrian

Edit: Extended test results (unfortunately, theyre bad)

Code:

Alert! Apple Hardware Test has detected an error.
4SNS/1/40000000: TCOH- 131.000

What should I do?

Click to expand...

Faulty Seagate 1 TB hard drives caused this fan problem on some iMacs. Apple is still replacing them. See this article: <http://blog.macsales.com/15165-apple-extends-imac-hard-drive-replacement-program>
and Apple's page: <http://www.apple.com/support/imac-harddrive/>
Hurry, their support could end any time.