This is off of this website, http://myweb.accessus.net/~090/cadidiag.html
" HOW TO "ENTER" DIAGNOSTIC MODE
To enter diagnostic mode, proceed as follows:
- Turn ignition "ON." Engine can be either OFF or RUNNING. For cars equipped with the Digital Instrument Panel Display (Digital IPC):
- Simultaneously press the "OFF and "WARMER" (red) buttons on climate control center (CCC) until all display segments (IPC and CCP) are illuminated For cars equipped with the Analog Instrument Panel Display (Analog IPC):
- Simultaneously press on the Climate Control Panel (CCP) the "OFF" and "PASS WARMER" (up arrow) buttons.
After the segment display test, the driver message center displays all set DTC, current and history, in the following order:
1. PCM DTC
2. IPC DTC
3. ACM DTC
4. SDM DTC
5. TCS DTC
6. PZM DTC
7. IRC DTC
8. RFA DTC
9. CCP DTC
10. MSM DTC
11. MMM DTC
12. PHN DTC
On the OBD II cars, the prefixes you'll see are: "IP" for the IPC instrument panel; "AC" for air conditioning module; "TC" for traction control; "RS" for the road suspension system; "SD" for the air bags; "PZ" for the body control module; "CC" for the cell phone; "RF" for the integrated radio system.
HOW TO "EXIT" DIAGNOSTIC MODE
To exit the diagnostic mode anytime during the test, depress the "ECON" or "DEFOG" button, or turn the ignition switch "OFF." Additional information on Trouble Codes and Diagnostic Procedures specific to the 1990 OBD-II Cadillacs.
First check the codes and write them down to see what they mean. Then clear them by going into diagnostics mode and when it says "PCM?", press the fan down button until you reach "RSS?" then press the fan up button to go into that submenu. After that press the fan down button again until you reach "RSS CLEAR CODES?" then press the fan up button and it will say "RSS CODES CLEAR" and you can turn the key off to exit diagnostics mode.
Hope this helps.