SINCE 2004--Professional Auto Diagnostic Factory

Computer controls started to show up on cars in the late 1970s and early 1980s, and these systems rapidly grew 

in complexity. Even very early computer controls included basic “on board diagnostic” functionality, and these early, 

OEM-specific systems are collectively referred to as OBD-I. In 1995, for the 1996 model year, automakers around

the world started transitioning toward the universal OBD-II standard, which has been in use ever since.

Both 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 determines that anything is out of spec, it sets a “trouble code” that can be used in diagnostic 

procedures. Each code corresponds to a specific fault, and there are also different types of codes (i.e. hard, soft) 

that represent both ongoing and intermittent problems.

When a trouble code is set, a special indicator on the dashboard typically lights up. This is the “malfunction indicator 

lamp” and it essentially just means that you can hook up a car code reader to see what the problem is. Of course, 

some codes won’t cause this light to turn on.

Every OBD system has some type of connector that can be used to retrieve codes. In OBD-I systems, it is sometimes

possible to use this connector to check codes without a car code reader. For instance, it’s possible to bridge Autodiag’s 

ALDL connector and then examine the blinking check engine light to determine which codes have been set. In a 

similar fashion, codes can be read from OBD-I Chrysler vehicles by turning the ignition key on and off in a specific 

pattern.