Hobbies Cars & Motorcycles Why You Can't Read Your OBD-II Codes It could be a blown fuse or dust in the port Share PINTEREST Email Print Getty Cars & Motorcycles Cars How Tos Basics Reviews Classic Cars Corvettes Mustangs Tires & Wheels Motorcycles Used Cars Trucks ATVs & Off Road Public Transportation By Matthew Wright 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. Learn about our Editorial Process Updated on 05/18/19 If you're scanning your car's computer for OBD (on-board diagnostic) codes and getting nothing in return, there are some things you should check before you give up and take your car to the shop. If you're resourceful enough to use your car's OBD system, then you're way ahead of the game, but let's start with a refresher on the OBD-II code's diagnostics, error messages, scan ports, and more. What Is OBD? Since the mid-1990s vehicles have had a built-in troubleshooting system known as OBD. A computer in your car monitors sensors that measure such things as engine temperature, exhaust gas mixture, and other metrics that would mean very little to you unless you're a serious automotive troubleshooter. The computer in your car or truck constantly monitors these sensors to be sure that the factors they measure all read within the manufacturer's optimum or safe range. If they go out of that range, the computer makes a note of it and stores this as an error code. In a modern car, there can be hundreds of error codes, each one pointing to a specific issue. A mechanic—professional or skilled do-it-yourselfer—can access these codes to measure the overall health of the engine. You do this by plugging a scan tool into a computer-style port on your car (your repair manual will show you where it is) and downloading the codes. Then you can go to a site such as OBD-Codes.com and see what the codes translate into. You can have your codes scanned free at most auto parts chain stores. Blown Fuse If you've plugged into your car's diagnostic port and aren't reading anything, you might think that your OBD-II brain has been fried, but don't declare it dead yet. The most common reason for getting no OBD code is simply a blown fuse. On many cars, the engine control module (ECM)—also called the engine control unit or electronic control unit (ECU) or powertrain control module (PCM)—is on the same fuse circuit as other electrical devices, such as the cigarette lighter or accessory port. The lighter is prone to blow fuses on some vehicles, and if there's no juice going to the ECM, it can't tell you what's going on. Even a fuse dedicated solely to the car's computer diagnostics can blow for no apparent reason. Check your fuses to see if any have gone bad. Remember that your car or truck might have more than one fuse box. This should be covered in your owner's manual or a proper service manual. Clogged Port Another reason for getting no reading is that the scan port has become clogged with dust after years of not being used. You shouldn't use a spray cleaner or anything else that could get the port wet, but wiping it with a soft cloth or blowing compressed air across it could help clear out anything that might be preventing your scan tool from getting a good reading. If one of these fixes works and you now know what codes your vehicle is storing, you can proceed with vehicle maintenance, at the shop or in your garage. If not, the shop might be your best bet. Sources "What is OBD II? History of On-Board Diagnostics." Geotab.com. "Symptoms of a Bad or Failing Engine Control Module (ECM)." Yourmechanic.com.