Background, On-Board Diagnostics

OBD I
1996 will be a year of major change for Saab. This change won't be readily apparent in the showroom but it's affect on the service department will be considerable. The source of this change is the government-mandated implementation of a new generation of on-board diagnostics, commonly referred to as OBD II. To better understand where we will be going, let us spend a few minutes reviewing where we have been.

The present generation of on-board diagnostics, known as OBD I, first appeared on 1988 Saabs equipped with the L.H. 2.4 fuel system. The California Air Resources Board (CARB) mandated the introduction of on-board diagnostics as a means of providing the service industry with the information needed to find and fix emissions-related faults. At the risk of oversimplification, here are some of the major characteristics and shortcomings of an OBD I system.

^ Not all circuits or components which could affect emissions are monitored.

^ An almost-complete failure must occur before a fault is detected. The systems lack the sophistication to "think" except in terms of black or white, on or off.

^ Fault codes may be read by triggering the MIL (Check Engine light) and, in later production, with scan tools such as Intellegant Saab Tester (ISAT).

^ A serious fault will turn on the MIL as soon as the fault is detected. Once the fault is no longer present, the MIL will be turned off at the next restart.

^ The location and appearance of diagnostic connectors can vary from model to model and year to year. Each manufacturer has their own unique terminology, diagnostic routines and test equipment. It has become difficult for anyone other than a "factory" trained and equipped technician to diagnose and repair these vehicles.





OBD II
The development of the standards which we now call OBD II began back in 1988, initially by CARB and later on as part of the 1990 revisions to the Federal Clean Air Act. The rest of this book is 'devoted to explaining just how OBD II works but here, as an introduction, are the basic objectives of the program.

^ To quickly identify vehicles with emissions related problems. Studies show that a significant percentage of our current air quality problems are caused by a few poorly maintained or malfunctioning vehicles.

^ To minimize the length of time between the occurrence of an emissions-related problem and its repair. Any significant increase in emissions will turn on the MIL (check engine light) and set a code. The driver will be warned that a problem exists and many state inspection programs will check for stored DTCs (fault codes). Cars with uncorrected problems won't have their registration renewed.

^ To provide assistance in the diagnosis and repair of emission' related problems. Technology has advanced to the point where traditional diagnostic methods are often inadequate. More and more often, only the control unit itself can judge the condition of the system.