94 G 20 no error or trouble codes flashing

I have a 94 G20 thats running rough,and has an intermittent check engine light that comes on and off, but when I jump the A-B terminals, the only code is 1-2 (12) or normal diagnostics mode. I was always under the impression that when a check engine light comes on it stores that code until its been cleared. Does the computer clear codes periodically? Anyone know? I have a feeling my issue is an EGR valve.