How does my check engine light work?

picture of check engine lightComputer modules and sensors monitor almost every aspect of your vehicle and driving experience. When something on your car reads out of its normal operating range it can trigger the check engine light to come on.

The computer system tries its best to adjust things and compensate for the problem. Sometimes it does this so well that you can’t tell that anything is wrong by the way the vehicle runs.

When a sensor reads out of range there are trouble codes that the computer system will store to help the technician understand where the problem is located. Occasionally these problems are very easy to find, but usually there is a complex set of diagnostic procedures that lead the technician to the problem. Many times even the most proficient technician with the best information systems and scanners available will spend hours researching a problem to pin point its source.

Pulling a trouble code from a computer system is the easy part. Testing and making sure that the right repair is made takes advanced training, expensive equipment, multiple information systems and frequently a lot of patience.

Many people think that a code will tell the technician what is wrong with the vehicle and then all he has to do is replace the component. Frequently a trouble code points to a system that has several components, electrical connections and even software programming that could produce the exact same trouble code.

Let’s pretend that you just came home from work and your 6 year old is just not acting like he feels well. As a concerned parent you start your diagnostic process with a test –you press the back of your hand on their forehead and become concerned that they seem warm. Then you move on to a more precise test and pull out the thermometer. Darn, sure enough the poor little guy has a fever. Well, while these tests are necessary, at this point really all that you have identified is a symptom, not a cause. That is a lot like what the “code” indicates about your car.

Technicians fight a similar battle as they approach diagnosing your vehicle. Their experience and knowledge lead them quickly to potential problems. The computer codes even help point the way, but now it is time to find out exactly what the cause is. There could be several causes, much like figuring out what has caused the fever in your little one.

This is where the real challenge and work begins as a technician finds the true source. You need to know if the fever is caused by the flu, a sore throat, or simply because little Bobby was running around a lot before you came home. Knowing just the symptom is not enough information.

This all becomes more complicated as some codes refer to recent events and some are stored  as historical codes. Their importance and relevance vary from vehicle to vehicle. A generic scan tool does not have the capacity to retrieve manufacturer specific codes as well as some historical information. Advanced testing requires expensive specialized equipment. Sound complicated? Yeah, it really can be and as cars become more complex the testing procedures do too.

Have a question about your car?

Just Ask –

Lynn Beckwith

That Car Lady


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