On-board diagnostic (OBD) systems first started showing up on cars in the late 1970’s and early 1980’s and these systems rapidly grew in complexity. These OEM-specific systems are collectively referred to as OBD-I systems as they have basic on-board diagnostic functionality. In 1995, for the 1996 model year, automakers have transitioned to the universal OBD-II system which is still in use today.

OBD-I and OBD-II systems work in essentially the same way in that they monitor a variety of sensor inputs and outputs. If the system finds that there is a problem with any sensor, a “trouble code” is set, which corresponds to a specific fault. There are different kinds of codes that represent both ongoing and intermittent problems and these codes light up a special indicator on the dashboard called a malfunction indicator lamp (MIL), also known as the check engine light.

Some OBD systems do not need a code reader to check fault codes, for example codes can be read from OBD-I Chrysler vehicles by turning the ignition key on and off in a specific pattern. In other OBD-I systems, and all OBD-II systems, trouble codes are read by plugging a car code reader into the OBD connector. This allows the code reader to interface with the car’s computer, pull the codes, and sometimes perform a few other basic functions.

Code Reader

After the OBD socket has been located and hooked up to, the car code reader will interface with the car’s computer. Simple code readers are actually able to draw power through an OBD-II connection, which means that plugging the reader in may actually power it up and turn it on as well.

At that point, you will typically be able to:

  • Read and clear codes
  • View basic parameter IDs
  • Check and possibly reset readiness monitors

The specific options vary from one car code reader to another, but at the bare minimum you should be able to read and clear codes. Of course, it’s a good idea to avoid clearing the codes until you have written them down, at which point you can look them up on a trouble code chart. Although car code readers are great at providing you with a jumping off point for your diagnostic procedure, a single trouble code can have any number of different causes. That’s why code readers are less useful than scan tools.

Diagnostic scanner tool (OBD-II)

The functionality of an OBD-II scanner depends on whether it is a basic “code reader” or a more advanced “scan tool.” Basic code readers can only read and clear codes, while advanced scan tools can also view live and recorded data, provide extensive knowledge bases, provide access to bi-directional controls and tests, and other advanced functionality.

All OBD-II scan tools offer some basic functionality, which includes the ability to read and clear codes. These scanners can also offer the ability to check pending, or soft, codes that haven’t activated the check engine light yet, and provide access to a wealth of information. Data from virtually every sensor that provides an input to the on-board computer can be viewed via an OBD-II scanner, and some scanners can also set up custom lists of parameter IDs (PIDs). Some scanners also provide access to readiness monitors and other information for instance car/manufacturer specific for different models as they would differ in code but remain the same in diagnosis.