How to Read and Understand OBD Codes

Getting the Codes

1. 
   Obtain an OBD-II scan tool. You can find OBD-II scan readers at many online and auto-parts stores. If you have a Bluetooth-enabled smartphone, you can download an app to interpret the data and purchase an OBD reader that will display the codes and explanations directly on your device.

   1. If your car/light truck is older than 1996 you will need to purchase an OBD-I scanner which are more vehicle specific and do not use the universal OBD-II coding system. This article concentrates on the OBD-II system.
   2. OBD-II constantly monitors the performance of your engine and Emissions Control System. It will turn on your Check Engine Light whenever a malfunction occurs that causes the vehicle produced emissions to be greater than or equal to 150% of the Federal EPA mandated limits.
2. 
   Locate the Diagnostic Link Connector (DLC) in your vehicle. This is a somewhat triangular shaped 16-pin connector that is commonly located underneath the left hand side of the dash near the steering column. If you have trouble locating the DLC, search for the location on the internet using your car's model and year, or refer to the owner's manual.
3. 
   Insert the scan tool connector or code reader into the DLC. Turn your ignition on, but do not start your engine. You will see the scanner begin to communicate with the onboard computers in your vehicle. Messages like "searching for protocol" and "establishing data transmission link" may appear on the scanner's screen.

   1. If the screen stays blank and does not light up, jiggle the connector to achieve a better contact between the scanner and DLC connector pins. Older cars in particular may have poorer connections.
   2. If you still aren't having any luck, be sure that your cigar lighter works. This is because the OBD-II system uses the cigar lighter circuit to provide voltage to the DLC. If the cigar lighter does not work, locate and check the appropriate fuse.
4. 
   Enter in your vehicle information. On some scanners, you will need to input your VIN as well as the make and model of the vehicle. You may also need to specify the engine type. This process will vary depending on the scanner.
5. 
   Find the menu. When the scanner finishes booting up, look for a menu. Select 'Codes' or 'Trouble Codes' to open the main Codes menu. Depending on your scanner and year of the vehicle you may be presented with a few systems such as Engine/Powertrain, Transmission, Airbag, Brakes etc. When you pick one, you will see two or more types of codes. The most common are Active codes and Pending codes.

   1. Active codes are live codes or malfunctions that are keeping your Check Engine Light on. Just because your Check Engine Light is off doesn't mean the code or malfunction disappeared, it just means that the code setting conditions haven't occurred for two or more operations of the vehicle.
   2. Pending codes mean that the OBD-II monitoring system has failed the operation of an emission control system at least once and if it fails again the Check Engine Light will be turned on and the malfunction becomes an Active code.