low octane = detonation. detonation = engine damage. the knock sensor is supposed to detect this and retart timing. if for some reason your knock sensor is bad then you're in big trouble. get your octane back up ASAP and pray.

the cel codes is probably:

P0300 — Random Misfire
P0301 71 Misfire Cyl. 1 Or Random Misfire
P0302 72 Misfire Cyl. 2 Or Random Misfire
P0303 73 Misfire Cyl. 3 Or Random Misfire
P0304 74 Misfire Cyl. 4 Or Random Misfire

note: if indeed one of these codes are set the cel will flicker during misfire.