OBD-I systems (like in your 1990 Legacy) don't have a readily accessible method for resetting the light like later OBD-II systems do. There's no simple button or procedure to do this.
To truly reset the check engine light, you need to:
1. Diagnose the problem: Find out *why* the light came on. This often requires a diagnostic scan tool specific to OBD-I systems (often a code reader designed for that specific year/make/model, or a more generic scan tool that can handle OBD-I protocols). These tools read the diagnostic trouble codes (DTCs) stored in the car's computer.
2. Repair the problem: Once you know the code(s), you can fix the underlying mechanical or electrical issue causing the light. This could be anything from a faulty sensor to a more serious engine problem.
3. Clear the codes (after repair): *After* successfully repairing the problem, you may need to disconnect the battery for a short time (10-15 minutes) to clear the codes. Some OBD-I systems require more sophisticated methods of clearing codes, sometimes involving specific jumper wire configurations within the diagnostic connector.
In short: There's no quick fix. Ignoring the check engine light is not advisable. Proper diagnosis and repair are necessary to ensure your car runs safely and efficiently. A mechanic with experience working on OBD-I Subarus is your best bet for a reliable solution.
