I have Gli 2012 model check engine light problem is common in corolla 2009 and above models, same problem of check engine light was with my car i took my car to the Toyota 3s dealer for the remedy of this problem they performed the IT test by attaching the scanner(in common mechanics language called computer) the error was of oxygen sensor which is installed in the exhaust pipe(silencer).....the mechanic there told me that it is very common problem on cng in this model.according to him when we drive the car on cng,the gas doesn't burnt completely due to that unburnt gas "oxygen sensor" sends the error msg to the ECM that fuel is not burning completely that's why check engine light get turn on.mechanic said me there is no need to worry about this light when ur car is on cng.