So....lets see if this is a hard code set or transitory. Disconnect your (-) battery terminal for 1 minute or more and then reconnect. Start the car and take it for a spin. If the SES light comes back on, then you need to make the repairs. I was able to find some info on-line that might be of help.
FIX for GM obd1 with code 42:
Remove the distributor assembly and replace the following:
Pick up coil
Once the above parts are replaced, reset the code and go for a quick road test to confirm. If your engine does not have a distributor, replace the crank sensor instead of the pick up coil. But check also for input voltage to the crank sensor harness wire. In some cases, the crank sensor magnet probe could be cracked and make sure to check them by removing the sensor for close visual inspection.