i had 2 codes last weekend (171 and 174, according to autozone maf and o2 sensor), and i wasn't able to it to the dealer. in the meantime i changed from e85 to premium and yesterday the engine light is gone. car is going to the dealer tomorrow; will the dealer know the codes were present? i dont want them to think i'm crazy.

 

Those codes may still be stored (possibly as 'pending' codes).