Apple Hardware Test (AHT) on upgraded iMac

I have 24" iMac, vintage early-2008. I have updated to MacOS 10.7.5, and have tried to run Apple Hardware Test (AHT) unsuccessfully, using directions from the Apple support page (ie; power up iMac with key D pressed and held) I also can't run it from the original OS media disk, OS/X Leopard. Any suggestions?

Thanks in advance!

Dean