AHT not working

You need to run AHT from disc two of your original gray install discs.

Try this

1) SHUT DOWN the computer (Don't RESTART... shut it down)

2) Hold down the OPTION + D keys

3) Press the Power Button (make sure to hold down the OPTION + D keys first)

4) Continue to hold down the OPTION + D keys until the system enters AHT mode

5) Let the app verify the hardware setup before initiating the test.

6) There are two testing modes. To select the "Extended Test" activate the check box below the Test button.

7) Press Test button to begin test

Find those discs.

One other option, look here to see if you can find your version https://github.com/upekkha/AppleHardwareTest

What error code did AHT provide? Remember that the HDD fan is controlled by circuitry in the hard drive itself, so don't be surprised if you need a new hard drive. Make a backup before you take it in.