'91 trooper cranks, won't start

So, sometimes it won't start, sometimes it will. The times it won't the 'check engine light' doesn't come on. Sometimes if I leave the key on
for a period of time, the light will eventually come on and it fires right up once I turn the key to start. The only flash code that the ECM gives is 12 which seems to be un-documented in this particular model.
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