The ECM which controls the SES light stores trouble codes in its memory for 50 engine starts (after which the error code is erased from memory).
To access the trouble codes in the ECM, you need to ground the "test" terminal on the ALDL (Assembly Line Diagnostic Link) connector. This connector is located under the dash on the driver's side, just left of the center, about where your right knee would be while sitting in the driver seat. The "test" terminal is pin "B", while the ground terminal is pin "A". The pin out is below (looking at the front of the connector):
(Have a pen/paper ready to write down trouble codes.) Take a short piece of wire and connect pin "B" to pin "A". I've found that a thicker wire (like that from a paper clip) will be held in the place easier. Place the ignition in the "ON" position, but with the engine stopped. The SES light should be steady on.
Once the "test" terminal has been grounded, the SES light should flash once, pause, then flash twice in rapid succession. This is code 12, which indicates that the diagnostic system is working properly. The Code 12 will repeat three times followed by the first error code stored in memory. The error code will be repeated 3 times and then change to the next error code (if any). Trouble codes will flash in numerical order (lowest code number to highest). The trouble code series will repeat themselves for as long as the "test" terminal remains grounded. Write each unique error code down. When done, remove the wire jumper and turn off the ignition.
If you want, you can clear the long term code memory by either removing the "ECM/BAT" fuse or disconnect the battery for greater than 10 seconds.
Once you have the trouble code(s) in hand, you can look them up easily on MikeD's web site. Check out: www.darklair.com/monte/codes.html
Hope this helps... good luck in your troubleshooting.