I would bring it in, because you can never be sure
Check this site for information: http://www.troublecodes.net/Subaru/
Code 33 means: Vehicle speed sensor (VSS) or circuit
If you have no spark on the 1991 Subaru Legacy, you should check the spark plugs and spark plug wires for replacement. You should also check the battery and ignition for replacement.
*Hopefully* the same as other Subaru engines, low down the left hand side about half way up the engine (facing the engine) there should be a short yellow loop. Check it when the engine is cold!
You need an OBDII computer a) to read the reason for the check engine light, and b) to reset the error code if you know what caused the light to come on. If you don't fix the condition that caused the light in the first place, it will probably occur again.
Belt was probably installed incorrectly. Take it back and have them check it.
Check for spark Check for fuel
Above the gas pedal, under the dash, there is 2 wires with clips that you plug together. Check engine light will flash the codes recorded in the ecu.
Check your thermometer and if that's not it...head gaskets.
It is possible that a Subaru Check Engine Code #12 is a bad starter switch. This is usually the starter relay switch malfunction code for Subaru.
Check the 1991 Legacy for a stuck thermostat. Check to make sure that the water pump is working properly.
clean the maf sensor with maf cleaner and general tune up, fuel filter and fix check engine and related problems