On Board Diagnostics II Theory

(This material is taken from Smog Check Advisory. By Adriane Chiu and Mike McCarthy, Air Resources Board)

Note: OBD = On Board Diagnostics,  MIL = Malfunction Indication Light ( also known as "Check Engine Light")

N.B. This page is for information purposes only. Since in South Africa there isn't a strong emission law some info may not be relevant!

OBDII, California's second generation of OBD requirements, is a diagnostic system incorporated into the vehicle's power-train computer that is designed to assist in pollution reduction and prevention.

OBDII has Three Main Objectives:

  1. To reduce high emissions caused by emission related malfunctions;
  2. To reduce the time between the occurrence of a malfunction and its detection and repair; and
  3. To assist in the diagnosis and repair of the malfunction.

The OBDII system accomplishes these by monitoring virtually every component and system that can affect emissions during normal driving, alerting the driver through a dashboard malfunction indicator light (MIL) and storing fault code information for technicians.


OBD I, adopted by California in1985, applied to 1988 and subsequent model-year vehicles and consisted of limited functional and circuit continuity checks of some components.

Although OBD I provided auto manufacturers with some experience in designing and implementing diagnostic systems, it was not a comprehensive system.

There was a lack of standardization (e.g., standardized connectors, scan tools, and fault codes), which resulted in many different manufacturer specific designs in the field.

Additionally, each manufacturer defined its own performance levels for how "bad" a sensor had to be before it would illuminate the "check engine" light, or MIL.

Furthermore, manufacturers were allowed to illuminate and extinguish the MIL at their discretion.

OBDII, adopted by California in 1989 (and later by EPA) and required on all 1996 and subsequent model-year vehicles, addressed the shortcomings of OBDII with the establishment of performance standards and a great deal of standardization.

OBDII systems expand the scope of monitored components and systems, as well as include more specific performance criteria for determining malfunctions (e.g., before tailpipe emissions exceed 1.5 times the Federal standard).

OBDII also requires vehicle manufacturers to use the same data link connector, communicate with multiple scan tools, and report information such as fault codes in a standardized format to ensure that all technicians (dealer or independent) have access to a minimum set of fault information.

General Requirements:

    The OBDII system monitors virtually all emission-related components and systems for malfunctions that can cause emissions to increase.

However, the OBDII system does not have a sensor in the tailpipe that turns on the MIL whenever emissions are high. Instead, the OBDII system monitors every component individually and turns on the MIL when any one component is clearly malfunctioning or when calculations indicate that malfunctions will cause the vehicle's emission to be greater than 1.5 times the federal standard.

This means the MIL is not going to come on just because a car is old and all of the components are partially deteriorated. Rather, the system will only turn on the MIL when a component, by itself, is clearly outside of design specifications.

Often, these malfunctioning components will cause increases in emissions, but not always. In some cases, it may take a combination of faulty components to cause emission increases.

Incorporating OBDII systems checks into the Smog Check program will ensure that motorists fix each of these faulty components routinely, rather than letting numerous faults go unrepaired until emissions are very high.

At such a point, it is much more difficult to correct all the faults, and the expense can be very high. Put more simply, it is more sensible to require motorists to maintain their vehicles properly to avoid more expensive and difficult repairs and keeping emissions at a minimum.

Given the competitiveness of the auto industry, they strive hard to save costs. Thus, from an emission perspective, if the manufacturer installed an emission related part in the first place, it is also worth fixing it when it no longer functions adequately. Sooner or later, faulty components adversely affect vehicle emissions and performance.

Components and Systems:

The components and systems monitored by the OBDII system can be divided into two general types:

  1. The major monitors
  2. The comprehensive components.
  1. Major monitors consist of the misfire, catalyst, oxygen sensor, exhaust gas recirculation (EGR), secondary air, evaporative leak check, and fuel systems.

    These monitors are required to detect malfunctions and illuminate the MIL generally before emissions exceed 1.5 times the applicable Federal Test Procedure (FTP) standards. The FTP is a special laboratory test that is required to be conducted by auto manufactures to show their vehicles comply with emission regulations before they are allowed for sale in California. The test simulates city driving after the vehicle has been parked overnight.

    The majority of OBDII monitors (e.g., all the individual sensors, valves, solenoids, etc.) fall under the "comprehensive components" category. This category consists of input or output components that can cause an emission increase or are used to monitor any other monitored components/systems (e.g., the major monitors).

    For example, if the catalyst monitor is designed to run only when the vehicle is within a certain vehicle speed range, the vehicle speed sensor needs to be monitored. If it wasn't monitored, the vehicle speed sensor could malfunction, the catalyst monitor would never run, and the system would never know if the catalyst was still working properly or not.

  2. Comprehensive components also include any component that, when malfunctioning, can cause an emission increase during any reasonable driving condition, whether it be idle, cold start, acceleration, cruising, or any other condition.

    So, even though a malfunctioning component may not seem to cause an emission increase during some conditions (like an ASM test), it probably does under other driving conditions. For all comprehensive components, the MIL is required to illuminate when any individual component is out of specification or fails to work when commanded.

    Generally, the OBDII system is required to illuminate the MIL after the same fault has been found in two different driving cycles, which helps to ovoid MIL illumination for random faults or abnormal conditions. The MIL is only allowed to extinguish when the same fault has not been detected on three successive driving cycles.

    Diagnostic Trouble Codes (DTCs) remain stored for around 40 driving cycles to make sure that information is still available to repair technicians even after the MIL is extinguished.

MIL Display Options:

Currently, the only acceptable display options for the MIL are as follows:

Manufacturers can only use the MIL for indication of emission related faults, and not for maintenance reminders, mileage intervals, or any other non-emission-related items.

Therefore, the MIL should not be confused with other lights manufacturers use for these non-emission-related items, such as "Service Vehicle Soon," "Maintenance Required," or other phrases.

Currently, some Smog Check stations in California have begun beta testing a software update that incorporates OBDII into the Smog Check program. Once beta testing is completed, all Smog Check stations with BAR-97 Emission Inspection Systems will be required to install this new software and hardware.

This transition is expected to be completed by the end of this year. Initially, vehicles will not be failed for OBDII fault codes unless the MIL light is on. During this period, ARB and BAR will be analyzing the data collected to help determine the most effective way to incorporate OBDII into the Smog Check program.



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