What is the year and model of your Buick? Any problems with how the engine runs right now?
When you say the car has been very dependable until that light started coming on - do you mean that the engine is acting up since the light started coming on, or just that the light itself is coming on?
The service engine soon (SES) light only comes on when the on-board computer detects a fault in engine driveability or performance, transmission control system or transmission, or the emissions system, and a fault code is set and stored in the computer. Faults that can set the SES light can range from a vaccuum leak, engine mis-fire, low engine compression, faulty sensor such as O2 sensor or other engine sensor, hydraulic pressure loss in transmission, transmission shift solenoid, open/short in wiring harness, emission evaporative system leak, or one of dozens upon dozens of reasons. Not all them will cause outward symptoms.
The only way to find out what is setting off the SES light is to read the onboard computer for stored DTCs(diagnostic trouble codes). On a 1991 Buick this can be done with either a diagnostic scanner at a shop, or you can retreive the codes yourself.
Below is how to retreive the codes yourself and what they mean:
To extract a trouble from the computer for diagnostic purposes, the Assembly Line Communication Link (ALDL) connector is used.
Ground terminals A and B with the ignition "ON" and engine not running. The system will enter the diagnostic mode. In this mode, the computer will display a 12, three consecutive times, by flashing the "CHECK ENGINE", "SERVICE ENGINE SOON" or "MALFUNCTION INDICATOR LAMP" or light. A 12 consists of one flash, a short pause, then two flashes in rapid succession.
After 12 is displayed, any stored trouble s will be displayed by flashing the "CHECK ENGINE", "SERVICE ENGINE SOON" or "MALFUNCTION INDICATOR LAMP" light. All s are displayed 3 times each then a 12 will repeat. In the diagnostic mode, the computer will also energize all computer controlled relays and solenoids.
The ALDL diagnostic connector is located Front seating area, driver side, under dash, mounted near fuse panel