If you haven't already BTDT, you might start with the "Virtual Debug Switch Settings: section of man prom
. Here's the cut to the chase part:
Virtual Debug Switch Settings
PROM boot behavior can be altered by changing the value of the virtual
debug switch. The value of the virtual debug switch can be displayed or
altered from either the system controller or from POD mode with the dbg
POD command. The values in the following list of virtual debug switch
settings are hexidecimal numbers. These values can be OR-ed together to
set multiple options.
Diagnostic Testing Level
0 Normal testing.
1 No testing.
2 Heavy testing.
3 Manufacturing-level testing.
Diagnostic Output Level
4 Verbose. Information level is set to verbose.
If you're interested, from there you could work your way into the "POD Commands"
and "Undocumented (POD Commands)"
articles in the nekowiki.
If that piques your interest (and you haven't already been inside the POD), the pod command "help" isn't a bad place to start. To ensure the system will still boot the power on diagnostics routine sometimes disables hardware with otherwise non-fatal errors. Clearing stale error entries from the POD log
can be another useful way of getting past problems.