If your malfunction indicator lamp lights up as "CHECK ENG", it usually points to the emission system and this warrants an engine scan in the casa or special auto repair shops to pin point where in the emission system is defective. In most cases the sensors are the ones at fault. The most common emission sensor that lights up the "CHECK ENG" is the MAF (mass airflow) sensor or in other engines the MAP (manifold absolute pressure) sensor. Other sensors that can light up the CEL are the oxygen sensor, TPS (throttle position sensor), IACV (idle-air control valve) and the EGR valve. others are too minor to mention like a leak in the vacuum hoses of the throttle body going to the MAP sensor. The engine scan (MUT -multi use tester) will narrow down and pinpoint the defective sensor as well as erase the error codes in your car's ECU. Illuminated CEL will not leave you stranded on the road but will affect your fuel consumption drastically. If you cannot have your car undergo an engine scan yet, you can have your MAF sensor cleaned sparingly with an electronics contact cleaner. Disconnect your battery while cleaning your MAF sensor..If after cleaning your MAF sensor removes your illuminated CEL, then your MAF sensor is just dirty and cannot do its job. But if the CEL does not disappear after cleaning your MAF sensor, the MAF sensor must then be defective and needs replacement or there are other problem in the emission system. This is where the Engine Scan now becomes very useful...