Two common causes for the check engine light coming on are that the ozone sensor needs replaced or the gas cap is damaged or missing. Other reasons why the check engine light may come on include a bad catalytic converter or a faulty mass airflow sensor. The best way to tell the cause of the light is to have a mechanic pull the codes.
Generally, when the check engine light comes on, the driver should take the car to a mechanic to have the error code pulled. By examining the code, the mechanic can immediately diagnose the problem and recommend repairs.
The oxygen sensor in a car is there to measure the amount of oxygen in the exhaust system. If this isn't working properly, the engine will go through fuel quickly, and eventually the sensor will cause damage to the catalytic converter and spark plugs.
If the gas cap does not fit properly or is damaged or missing, the car will lose fuel through evaporation, causing increased fuel consumption.
When the catalytic converter is not working properly, the vehicle will not pass emission inspections and the car may run hotter than normal. Fuel economy is affected, and drivers usually notice reduced performance.