Check the archives on how to check the ECU for fault codes. I believe a 91 has the button and LED set-up near the X-11 port or something like that. Maybe only California 91s have the feature.

Put the key in the on position (engine not running), hold down the button for 2 seconds, count flashes, repeat until you're back to a number you've seen. Hold the button down for 10 seconds after each code to delete the code once you've noted it.

Sixto
95 S420
87 300SDL