If you have check engine codes or your check engine light is on (also known as a MIL...I will refer to it as that on this site), then there is a reason. Many people think that if your Malfunction Indicator Lamp (MIL) is on but your car still runs fine then it is nothing to worry about. They are wrong. That light could be costing you money.
If you notice your MIL light come on that basically just means that the computer has noticed a problem in one of your cars systems. When your car is running it monitors many different sensors and circuitry. Over time these sensors and wiring degrade and eventually do not work correctly. When this happens your cars computer will see this and alert you that there is a problem.
Not all codes will result in your car running poorly...at least for now. Some codes set because of environmental concerns, mainly that gas fumes being released or problems with the cars exhaust. This means that your car might run just fine, but there is still a problem that needs to be checked and diagnosed.
On many older cars it was very easy to clear the check engine codes. All you had to do was disconnect a battery cable. Well, this will generally no longer work on cars that were built after about 1997.
For these cars the only way to clear the codes and reset the check engine light is to use a scan tool. In fact, you will probably be doing way more damage than good to remove a battery cable as your cars computer, radio, etc. will lose all of it's saved information and you will either have to input it all or in some cases the car will have to re-learn it's operating characteristics.
Many people think that the code will tell you exactly what the problem is. This is one of the most inaccurate beliefs that people have about their car.
A check engine light code means that the computer has noticed a problem in aSYSTEM. In order to tell what is causing the code ALL parts of the system must be checked and tested to see if the problem is with the sensor, wiring, computer or even with another, related sensor or system.
This diagnosis can take a few hours. Many times it will not take very long to diagnose, but other times it will take a long time. The computer code will only alert the technician to the general area of the problem.
If you get your check engine codes read somewhere or if you do it yourself you should realize that just replacing the sensor for that code is taking a big chance and may not fix anything.
It is almost always better to take your car to your mechanic to have them diagnose what is going on and replace the needed part. this way there is no wasted money and you know exactly what the problem was.
To learn more about specific codes read my OBD codes page.