When you open the hood of the car, there are plenty of electrical systems. Both new and used cars are equipped with these electronics. The engine control module (ECM) uses actuators and sensors in order to get the best fuel economy and performance.

Software will enable the ECM to adjust for any driving scenario. Since electronics can’t last forever, the check engine light (CEL) is the system’s way of letting you know that something is wrong.

Sometimes the problem is more obvious and the CEL comes on when you know there is a problem. Other times it may not be so obvious, and the problem only means a decreased fuel economy, which many drivers don’t monitor regularly.

What to Do When Your Check Engine Light Comes On

Once the CEL comes on, you will need to figure out why it is illuminated. There is no way to know why the light came on unless you or someone else you know has access to a scan tool and the knowledge to diagnose the problem.

Auto part stores may sometimes offer free diagnoses for the CEL. A local repair shop might charge about $100 for the diagnosis. The scan tool will reveal one or more of the diagnostic trouble codes. Some of these codes are easy to interpret, while others don’t really give a direct answer.

This can be a problem when it comes to free scanning or do-it-yourself scans. For example, an oxygen sensor code doesn’t necessarily mean that you have to replace the sensor.

Even if you have to pay for a diagnosis, this is your best way to figure out what is truly wrong with the car. Just knowing the code won’t help much and the best people get paid because they have put in thousands of hours to learn more about used cars and trucks, auto diagnostics, and auto repair.