Coolant that needs changing will not cause a CEL. The light is not programed to come on at any interval, it only comes on when the computer detects an operating or self-test parameter that is out of range.

Likewise, the octane of your fuel cannot cause a CEL by itself. These engines were designed to run on 87 octane. Consult your owner's manual for clarification.

The only way to determine why the light came on is to read the code(s) as BillyBob69 suggested. If you take it to a chain store, make certain that you right down the EXACT code number(s). They will be in the format of PXXXX where XXXX is a 4-digit number including leading zeroes.

Once you have the code(s), come back here and perform a SEARCH using the code(s) as your search keywords. If you need further help after researching the code(s), just ask.

Steve