Stop-a and at the OpenBoot prompt type:ok? setenv diag-switch? true
Now for the catch. When you turn the diag-switch? on the output is printed in the serial console. This means the first serial port. If you don't have a serial console, and you're using the system with a monitor and keyboard, you'll find yourself staring at a dark screen for a couple of minutes while the diagnostics run.
It's really easy to think the machine is busted if you don't know what's going on. And that is precisely what happened to me.
Now for the punchline. Apple's computers also use Open Firmware, the incantation is
Option-Command-o-f. Once you're in Apple's OpenFirmware prompt, you can also turn diag-switch? on. Guess what happens... yup, a few minutes staring at a dark screen thinking the machine had died. Only Apple computers don't really have a serial port.
