Cadillac Repair: Service Engine Soon Light/1993 Cad. Eld. 4.9l, vacuum lines, onboard diagnostics

Question
My check engine light is on continuesly.  I went through the climate control by turning the ignition on with engine off and holding the Off and Warmer buttoms at the same time for 2 seconds.  Here is what shown on the display:
PO48 Current;No IPC Code;NoACP Code;NO SIR Code and PCM?
What does that mean?    Also, I got the information 'Set Timing Mode' when I checked the diagnostic plug with the little metal code reader pin.  What does that mean also? Thank you...128,500 miles on car.

Answer
P048 = EGR malfunction. Check all your vacuum lines to the EGR valve and hand test the valve. You should also remove the valve and clean it.

CHECK the valve FIRST to make sure it moves during engine rpm increase.

'Set Timing Mode' Don't worry about it .

Why don't you use the onboard diagnostics via the climate control panel instead of using the ALDL plug ?.