How to Obtain and Interpret Diagnostic Codes

My career as an automotive improvement expert began when I modified my first car. Since then, as a car enthusiast and published author, my obsession with the automobile has led me to enhance tons of vehicles both inside and out. That’s why I’m thrilled to be a part of a brand new series designed to help keep your car looking great and your engine running like new! I’m Chris Duke, and welcome to eHow’s Auto Center brought to you by Mobil1! Hey Chris, the check engine light is on in my car and I really want to try and diagnose this myself. What do I need to do? -John John, those check engine lights are the worst, aren’t they? Unless you’re dead on the side of the road it can be really hard to figure out what exactly is wrong with your car. Thankfully, all cars since 1996 use an industry standard On-Board Diagnostic system that you can check to see exactly what is causing that check engine light to light up. You will hear this system referred to as OBD-II, the II part being the latest industry standard, when mechanics and other people from the industry talk about it. If your check engine light is on you can always stop by your local auto parts store. Most shops can read the codes for free. The most important parts of the vehicle's operation are monitored by the check engine light. Most are designed to come on for a short period of time and then go off again if the problem is considered minor. If it remains untreated, it could become serious, however. Since you’re going to do this yourself here are several different ways to get the diagnostic error code for your problem. The most reliable are some of these aftermarket code readers that simply plug in to the OBD-II port in your car. They will read the code from the cars computer and display it on a built in screen for you to reference. Some will even let you reset the code when you’ve completed a repair or just need to test if the code will come back, but more on that in a bit. Once you have your OBD-II reader or ‘scanner’, you simply plug it in to the OBD-II port, which is usually found somewhere on the lower dash on the driver’s side (check your owner’s manual for more information on its location), turn the ignition switch on and use the scanner to pull up the code… So now you have your code or codes… Great! But what do you do now? Well while each manufacturer will have their own set of specific codes, there are a large number of industry standard codes that will quickly tell you the area of concern. Most code readers either tell you what the code means or come with a book, but there are several ways to find out how to interpret your code(s) ranging from looking them up on the internet, asking your local mechanic, or even calling the parts or service department of your local dealership. Also, if you have a repair/reference manual for your car (which you ALL do… right?) You may be able to find them in there. Ok, so we have our code.. which in this case is shown as P0100. We check our source and see that a P0 code is an OBD-II standard code and not manufacturer specific. We search down to P0100 and see the code means “Mass or Volume Air Flow Circuit Malfunction”. If needed we do a little more research and find that this means that either the Mass Air Flow sensor is either dirty, disconnected, has a bad wiring connection, or the sensor itself is faulty. A lot of times the sensor is just dirty. A quick clean is a lot cheaper than replacing a working sensor. When we look at the MAF system on our car and find that the MAF sensor plug has come loose. This can happen sometimes if your air filter box has to be removed completely when changing or inspecting your air filter. We plug the sensor back in and check that it is seated correctly. Then we go to our scanner and tell it to ‘reset’ the code if it has that function. If your scanner doesn’t have a reset feature, you may have to remove a battery terminal to reset or ‘clear’ the code. Now that we have reset the code, we start the vehicle and look to see if the check engine light is still on or comes back on right away. No lights here, so we are good to go! Now in the case of a more advanced code or a code that lists an item that needs more in depth trouble shooting, check with your local mechanic for the best advice on how to tackle your issue. Being able to check your own trouble codes can save you a lot of time and money. It can also help you keep an eye on the other systems in your car to make sure they are working properly. It is a great skill to learn! Well, I hope that I’ve given you the know-how to tackle that check engine light! Don’t forget… when in doubt ask your mechanic, they are a great source of information! Thanks for the question John! For eHow’s Auto Center, I’m Chris Duke. Here’s to helping keep your car running like new!