A Guide to Reading Automotive Diagnostics

The inside of a car.
What You'll Need
Diagnostic Scanner
Diagnostic code reader (for instant readings)
Diagnostic Software
Laptop or desktop computer

The simple logic behind an automotive diagnostic system is that a technician can solve a car problem much more quickly if the car’s on-board computer tells him where to look first. Beyond this simple logic, however, a diagnostic system can offer you literally tens of thousands of different codes for the specific area you are trying to examine. Still, the diagnostic system (also known as the OBD or On Board Diagnostic) can be a tremendously useful tool for those who know how to use it properly.

Step 1 - Identify Version of OBD

The OBD was designed in the 1980s to be included in every car manufactured in the United States. If your model was made after January 1996, then your car should be operating under the OBD-II system (although some cars made in 1996 are not fully compatible with today’s equipment and software). Companies like GM and Ford use specific OBD-II protocols (SAE J1850 VPM and SAE J1850 PWM, respectively). Check the manufacturer’s handbook to find out exactly what type of OBD-II you have.

Step 2 - Do a Quick Check-up Before Heavy Diagnostic Work

If you have seen the “check engine” light on, you are likely concerned that something is seriously wrong with your car and that a diagnostic should be performed immediately. While this may be the correct course of action, it nevertheless would be a good idea to perform a routine check up before you begin heavy diagnostic work. In fact, the “check engine” light is supposed to come on if anything is wrong with your car. Checking for little things like making sure the gas cap is on or making sure the engine isn’t wet can save you hours of frustration later on.

Step 3 - Set up a Connection and Run the Software

All cars made after 1996 will have a diagnostic connection located by the dashboard. Use that connection to plug in the scanner. From here, attach the scanner to a USB port on your laptop or desktop computer. Run the diagnostic software as you would any other program. Choose a good diagnostic software to analyze the readings from the scanner. Today’s software should be equipped to analyze the data from the scanner automatically. The software should also be compatible with your car’s brand name (as each brand name has a slightly different diagnostic system). If you want a quick reading, use a code reader instead. The code reader doesn’t require software and will instantly tell you how severe the problem(s) is/are (although it won’t give you other information).

Step 4 - Look up the OBD Code and Interpret the Problem

If the software functioned correctly, it should display an OBD code, such as PO301. This code will tell you specifically what’s wrong with your car. For example, PO301 means that there has been a misfire at cylinder 1 detected. Once you have diagnosed the problem, you will have a much clearer idea of how to begin fixing your car (or how to let a professional mechanic do it for you).