Cadillac Repair: 1990 Cadillac Check Engine Light, letters and numbers, apparent reason

Question
I recently bought a 1990 Cadillac Sedan de Ville. It runs well but the "Check Engine" light comes on occasionally for no apparent reason. How do I run the diagnostic?

Answer
1. Ignition ON. Engine OFF.

2. Simultaneously push and hold the OFF and WARMER buttons for 3 seconds and release them.

3. Codes in the form of letters and numbers will appear and then stop.

4. write down EVERY LETTERand NUMBER that you see in the EXACT order in which they display and let me know what they are.

5. Turn OFF ignition.