Hobbies Cars & Motorcycles Understanding OBD or On Board Diagnostics and Engine Codes OBD Diagnostics Defined For You Share PINTEREST Email Print If your Check Engine Light pops up, you should download your OBD Codes. Virginia Star/Moment / Getty Images Cars & Motorcycles Cars How Tos Buying & Selling Basics Reviews Tools & Products Classic Cars Exotic Cars Corvettes Mustangs Tires & Wheels Motorcycles Used Cars SUVs Trucks ATVs & Off Road Public Transportation By Matthew Wright Matthew Wright has been a freelance writer and editor for over 10 years and an automotive repair professional for three decades specializing in European vintage vehicles. our editorial process Matthew Wright Updated June 25, 2018 On-Board Diagnostics (OBD) Codes may seem like the secret language of auto mechanics, but they're actually relatively easy to translate. At some point, the powers that be decided that it would be easier to figure out what was wrong with your car if it just told them what the problem was. Sounds like a great idea, right? In some ways it was, but there are 10,000 different diagnoses your car can give, so putting your finger on the right one can be difficult. What Could It Mean? Figuring out how to fix the problem is another kettle of fish, but the first step is to know what your car is trying to tell you. The language it speaks is OBD, which stands for On-Board Diagnostics. If something goes wrong, a technician (or you if you have a $59 code reader) can plug into your car, and will get a numeric code telling them what is amiss. Often your car will let you know ahead of time when something is screwy by turning on the Check Engine Light. Unfortunately, there are dozens of unimportant reasons for the light to come on—things like a loose gas cap. But checking the code will help you determine how seriously to take that light. If your Check Engine light is on, do a quick Check Engine Checkup to be sure it's nothing common and meaningless (which translates to "easy to fix for free.") These days, you can read your own OBD codes using a simple smartphone app! There is an initial investment, as the app is shipped with a Bluetooth enabled device that plugs into your car's information data port where it sucks the codes from the vehicles computer and transmits them to your phone. But these apps are reasonable in cost and can provide some great information. My favorite, the BlueDriver, includes all sorts of diagnostic advice along with giving you just the code that it's reading. If you want the info at your fingertips, an app-based tool is the way to go. The On-Board Diagnostics system your car is equipped with works on many levels and is a complex beast. There are often dozens of sensors that are constantly sending information back to the ECU (your car's central computer, also known as "the brain."). The ECU's job is to take all of this information and use it to keep the car running its best by compensating for anything that might now be operating at full capacity, or for conditions that are changing while the engine is running. It can do this in real time in most cases. Conditions that change often include air temperature, humidity, fuel quality, and engine wear. If your spark plugs are hideous, your engine's ECU can actually compensate for this by changing a few parameters. Of course, there's a limit to how much the computer can change, but the fact that it can make adjustments in fuel or timing on the fly make the system pretty impressive. If something does go wrong, not only does the ECU do its best to compensate for it, it records the even and stores it as an OBD Code. There are plenty of codes that can potentially be stored in the car's computer, and sometimes downloading the codes will leave you with more than one code to research and diagnose. Some codes indicate major issues, other can be a reflection of a single moment in time that has come and gone, leaving no damage and no trace of what went wrong except this error code you're trying to figure out.