Here are the Instructions:
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:
The RSS, CCP, MSM, MMM and PHN DTC appear only on vehicles equipped with these options. Press the FAN DOWN button in order to bypass the DTC display.
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.
when you have pen and paper ready to write them down then go here:http://myweb.accessus.net/~090/dtcobd2n.html#gmd