whats with the engine light always on? on 2000 Pontiac Sunfire

i believe its been on since i got it but i dont remember that far back but anyways even though we have bought a new gas cap its still shows the same problem from autozone they said 9/10 its the cap even though we have replaced it before that we have reset it with unplugging the battery its almost was off for a week someone said it could be the fuel injectors but i mean im getting 25-30mpg

Asked by for the 2000 Pontiac Sunfire