Download one and try it out.
If that does not work download another.
There's only four iMac10,1 files there.
Restore the image to a flash drive. Each download is likely less than 10MB, expanding to a disk image less than 200MB.
The hardware test would do a general test of the logic board, the chipset, and a cursory test of internal storage. The bulk of the test (particularly on the extended test) is for the RAM. The other tests flash by very quickly by comparison, likely less than 5 or 10 seconds out of the longer RAM test. The amount of memory installed is the main factor deciding the duration of the test.
An Apple store, or an authorized service provider, would have a more comprehensive test suite to run.
You could also search for a download for the "Apple Service Diagnostics 3S138", which is the correct version for a Late 2009 iMac. That's Apple's proprietary hardware test, which does a few extra tests compared to the Hardware test.
I ran the Apple Service Diagnostic 3S138 three times and got two errors.
The first was:
Hard Drive (test #4) - Turn Fan On Automatically - System turns on Motor and veriﬁes Motor reaches min/max values, +/- 10 percent threshold [19:23:52] ERROR -- 9 [Motor unable to stabilize between min and max limits] -- (1364) TEST FAILED [05/03/16 19:23:52] * TESTING FAILED [Elapsed time = 00:03:43] *
That error only happened once in the 3 runs.
The 2nd error was:
Temperature (Tp3H) -- Power Supply Primary Heatsink (test #1) - Sensor Reads Within Operating Range - Check to ensure that sensor reads within operating range. [19:48:42] ERROR - -13010 [SMC error TM_TEMP_UNSTABLE, the temperature read from sensor is unstable.] -- (1.00) TEST FAILED [05/03/16 19:48:42] * TESTING FAILED [Elapsed time = 00:00:01] *
This error happened all 3 times so I'm assuming it's the more serious. Anyone have any suggestions about how to go about rectifying this?
What issues are YOU having with your iMac?
If you can tell us about your actual symptoms, then someone may either have previous experience, or the test errors that you have may (or may not) be related to your symptoms.
I would call your two errors:
#1: intermittent hard drive cooling fan speed (maybe a failing hard drive cooling fan )
#2: Unstable temp sensor reporting from the power supply heat sink. (likely a failing power supply)
The video card is acting extremely strange. It will randomly shut off then it will come back after a few minutes, usually about 5. Which is how long I have it set to turn off for non use. I think it re-initializes. At first it only happened in Windows 10 and rarely, so I thought it was a software/driver issue. Now it happens so often that it make Windows unusable. When it’s happening in Windows it will carry on doing it if I boot directly into OSX. But after awhile it will stop doing it in OSX.
What I think is happening is that the video card is overheating in Windows and remaining overheated if I boot directly from Windows to OSX but it will eventually cool down in OSX.
Late 2009 27” iMac (All updates)
Replaced optical drive with 512GB SSD about 2 years ago.
hmm. 7 years old, and might be ready for a good cleaning inside.
Make sure the cooling air inlet on the back is clean and clear.
That's the circular grill underneath the hinge, and above the power connector.
Because of its location, that cooling inlet is easy to miss. It supplies air for the hard drive fan.
Other air passages would be inside, requiring partial disassembly, removing the LCD panel for some access to the internals.
Video card heat sink is cooled via the airflow through the optical drive fan.
The CPU (and the power supply, too) is in the cooling path for the third fan, the CPU fan.