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EGR system and Chevron 91

I have 1 92 Toyota Previa LE. The engine light came on. The only code stored was for the EGR system. A friend suggested using name brand 91 octane with no ethanol. After this Tx the engine light has mostly stayed off and the van just passed CA Bay Area smog.



Is this all a coincidence, the 91, the Chevron, the engine light staying off, or is there some cause and effect here?