Understanding On Board Diagnostics (OBD)

The purpose of the on-board diagnostic system on vehicles is to ensure the emissions control system and other engine-related components are operating properly. The OBD test will replace traditional exhaust tests for most 1996 and newer model vehicles. Recently passed legislation allows the Illinois EPA to conduct OBD tests and use those test results to pass the emissions test. OBD tests have been performed on an advisory basis since December 1999.

The OBD test will be phased-in because of the time needed to change the computer software and to accommodate the legislation. The OBD full implementation will take place over an 18-month period in three phases:

What is OBD and how does it work?

Since the early 1980's, many vehicles have used electronics and on-board computers to control many of the engine control systems, such as fuel and ignition. Vehicle manufacturers had to develop ways to diagnose problems generated by the new electronic hardware found under the hood. Thus, auto manufacturers developed the first on-board diagnostic (OBD) systems as electronic systems replaced mechanical systems. A more sophisticated on-board diagnostic system was eventually developed, and the second-generation on-board diagnostic system
(OBD-II) became required for 1996 and newer vehicles.
The engines in today's vehicles are largely electronically controlled. Sensors and actuators sense the operation of specific components (e.g., the oxygen sensor) and actuate others (e.g., the fuel injectors) to maintain optimal engine control. An on-board computer, sometimes known as a "powertrain control module" (PCM) or an "engine control module" (ECM), controls all of these systems. The on-board computer is capable of monitoring all of the sensors and actuators to determine whether they are working as intended. It can detect a malfunction or deterioration of the various sensors and actuators, usually well before the driver becomes aware of the problem through a loss in vehicle performance or drivability. The sensors and actuators, along with the diagnostic software in the on-board computer, make up what is called "the OBD system."

What is the connection between OBD-II and vehicle emissions?

OBD-II is an emissions control system. Older emission tests that collect or sample the exhaust produced by a vehicle identify vehicles that are already excessive polluters. However, OBD-II is a shift to pollution prevention. OBD-II can identify problem(s) with the emissions control system before the vehicle becomes an excessive polluter, allowing time to repair the vehicle before emissions increase. Left un-repaired, further damage can occur and emissions will increase. There are circumstances under which the vehicle computer will detect a system problem before the driver notices a driveability problem. Furthermore, OBD-II can detect problems that may not be noticeable upon visual inspection because many component failures that impact emissions can be electrical or even chemical in nature. By detecting these emission-related failures and alerting the driver to the need for potential repair, vehicles can be properly repaired before emissions become a problem.

How does OBD-II inform drivers of problems?

When the OBD-II system determines that a problem exists, a corresponding "Diagnostic Trouble Code" (DTC) is stored in the computer's memory and a special lamp on the dashboard called a Malfunction Indicator Lamp (MIL) is illuminated. This lamp is reserved for emissions problems only and cannot be used for other failures. Automobiles use a variety of warning lamps to notify drivers of different conditions. In the case of an emissions MIL, either the phrase "Service Engine Soon" or "Check Engine" is used or an engine symbol is displayed. This light, usually yellow in color, serves to inform the driver that a problem has been detected, and vehicle service is needed. When the car is delivered to the repair shop, a service technician can quickly retrieve the stored diagnostic trouble codes from the computer memory of the vehicle. The stored diagnostic trouble code(s) will help the service technician diagnose and repair the problem.

It is important to note that an illuminated MIL is intended to inform the driver of the need for service, NOT of the need to stop the vehicle. However, service should be sought as soon as possible. Drivers also may wish to consult a repair shop or their vehicle owner's manual for further guidance.

Why does the MIL blink or flash?

Under certain conditions, the MIL will blink or flash. This indicates a rather severe level of engine misfire. When this occurs, the driver should reduce speed and seek service as soon as possible. Severe engine misfire over only a short period of time can seriously damage emission control system components, especially the catalytic converter, which is typically the most expensive to replace. Drivers should also consult their vehicle owner's manual for manufacturer specific information.

How is the test performed?

There are two basic steps to the OBD test. With the key on and the engine off, the MIL is checked to verify that it works; and a cable is attached to the on-board computer connection (called a Data Link Connector or DLC) that checks the OBD-II system status.

Three pieces of information are downloaded from the vehicle:

What are Readiness Monitors?

Readiness monitors are indicators used to find out if emissions components have been evaluated. In other words, if all monitors are set to ready, the emission components have been tested. In some cases, complex driving patterns need to be followed to complete these monitors.

How can the Malfunction Indicator Lamp (MIL) be turned off?

After fixing the problem, the service technician will turn off the MIL. There are also situations under which the vehicle's OBD-II system can turn off the MIL automatically if the conditions that caused a problem are no longer present. If the OBD-II system evaluates a component or system three consecutive times and no longer detects the initial problem, the MIL will turn off automatically. As a result, drivers may see the MIL turn on and then turn off. For example, if the gas cap is not properly tightened after refueling, the OBD-II system can detect the vapor leak that exists from the cap not being completely tightened. If the gas cap is subsequently tightened, the MIL should be extinguished within a few days. This is not an indication of a faulty OBD-II system. In this example, the OBD-II system has properly diagnosed the problem and accordingly alerted the driver by illuminating the MIL.

My car didn't fail, but my Readiness Monitors are not set. How can I set them?

There are many reasons the monitors could be set to "not ready". Usually this is caused by routine maintenance. For instance, if the battery is disconnected for any reason, the monitors of most vehicles are reset. Also, a service technician may have to reset them as part of the repair process. Generally, the car must be driven to reset the monitors. Some manufacturers advertise driving procedures while others do not. The vehicle manufacturer or a qualified service technician is the best source for this information.

How does OBD-II help the environment?

The intent of OBD-II systems is to ensure proper emission system operation for each and every vehicle and light truck during its lifetime by monitoring emission-related components and systems for malfunction and/or deterioration. An important aspect of OBD-II is its ability to notify the driver of a problem before the vehicle's emissions have increased significantly. If the vehicle is taken to a repair shop in a timely fashion, it can be properly repaired before any significant emission increase occurs. OBD-II systems also provide automobile manufacturers with valuable feedback from their customers' vehicles that can be used to improve vehicle and emission control system designs.

How does OBD-II help consumers?

OBD-II systems are designed to alert drivers when something in the emission control system begins to deteriorate or fail. Early diagnosis followed by timely repair can often prevent more costly repairs on both emission control systems and other vehicle systems that may affect vehicle performance such as fuel economy. For example, a poorly performing spark plug can cause the engine to misfire, a condition sometimes unnoticed by the driver. This engine misfire can, in turn, quickly degrade the performance of the catalytic converter. With OBD-II detection of the engine misfire, the driver would be faced with a relatively inexpensive spark plug repair. However, without OBD-II detection, the driver could be faced with an expensive catalytic converter repair in addition to the spark plug repair. Furthermore, manufacturers have increased incentive to build higher-quality vehicles with better performance, reduced emissions, and more efficient powertrains to prevent problems that can lead to OBD-II detection. OBD-II systems also provide far more information than ever before to help auto technicians diagnose and properly repair vehicles during their first visit to the repair shop, saving time and money for consumers.

Are OBD-II related repairs covered by warranty?

US Federal law requires that the emission control systems on 1995 and later model year vehicles be warranted for 2 years or 24,000 miles. Many automakers provide extended warranty coverage beyond what is currently required by federal law. Federal law also requires that the on-board computer and the catalytic converter on 1995 and later model year vehicles be warranted for 8 years or 80,000 miles.



Data link connector


Diagnostic trouble code


Engine control module


Malfunction indicator light


On-board diagnostics


Second generation on-board diagnostics


Powertrain control module


Automotive Equipment - in tune with the Future

Crypton Diagnostic Equipment - Automotive Diagnostic Workshop Equipment as used by Dealers and Garages

Crypton Diagnostic Equipment Home