If your Check Engine light is on, it means your vehicle has experienced some type of emissions-related problem. It might be a minor issue or it might be a more serious issue. The only way to know is to connect a scan tool to your vehicle's diagnostic connector to read out the Fault Codes that have turned on the Check Engine light. Once the codes are known, the next step is to proceed with additional diagnostics to nail down what exactly is causing the problem. A code by itself will NOT tell you which part or parts need to be replaced. For more information on diagnosting Check Engine light problems, see the articles below.
A couple of things to keep in mind when pulling codes from a vehicle. Number one is write down the codes. Number two is do NOT erase the codes until you have written them down and performed any additional diagnostics that may be required to identify the problem. If you erase the codes with a scan tool, they will likely return and turn on the Check Engine light again if the original problem has not been corrected.
Also, erasing codes when a problem is still present will NOT allow you to pass an OBD II plug-in emissions check. The reason why is because all OBD system self-monitors have to run and complete WITHOUT resetting any codes before your vehicle can pass an emissions check.
One of the most common causes of a Check Engine light is a loose or missing gas cap. If your Check Engine light came on a day or two after filling your gas tank, check the cap to see if it is tight or missing. If the cap is loose, retightening the cap should cause the light to go out after several successive drive trips or within a day or two. For more information on this type of code, see Loose Gas Caps.