Cadillac Repair: service engine soon light, power control module, diagnostic test

Question
Hi i have a deville 1992 4,9L the "service engine soon" light has come on.  I just performed the diagnostic test on the ECC " Off and Warmer buttons"  i get no codes only "---". Is the OBD malfunctioning ?
Do you have any suggestions?

Answer
This is a fault in the DATA LINE between the PCM computer and the INSTRUMENT PANEL.

Has there been any work performed on the car lately or has the speedometer cluster been replaced ?.

If the --- is shown in the fuel data display it is in most cases a faulty POWER CONTROL MODULE (PCM), But NOT always.