Why is Apple hardware test (AHT) not loading when pressing key D on system start?
Pressing &#x2325;+D right after pressing the power button and before the gray screen appears works on my late-2011 15" MBP. This forces an internet-based hardware test, which will work even if you've lost the the local hardware test software. (It will probably work over WiFi, but you have more chance of success with ethernet.)