2009 Toyota Prius 109,000 miles check engine light come on

There are a number of reasons that the check engine light would come on in Your 2009 Toyota Prius -- in essence, it acts as a catch all for car related concerns. A check engine light will illuminate for a loose fuel cap, one of many failing sensors, a bad fuel pump, defective oxygen sensors, or worn spark plugs, to name a few. There's no real correlation with mileage, so the 109,000 miles is just coincidence. To pinpoint Your check engine woes, You'll need to check for diagnostic trouble codes (DTCs) with an OBD-II scan tool. Then, search Vehicle History for the code You're getting, and ultimately You should be able to decipher it. Sorry I couldn't be more help, but You may have any one of over a hundred different issues.