OBD-I and OBD-II Overview

The California Air Resources Board (CARB) began regulating OBD systems for vehicles sold in California beginning with the 1988 model year. The initial requirements, known as OBD-I, required identifying the likely area of concern with regard to the fuel metering system, exhaust gas recirculation (EGR) system, emission-related components and the powertrain control module (PCM). A malfunction indicator lamp (MIL) was required to illuminate and alert the driver of the concern and the need to repair the emission control system. A diagnostic trouble code (DTC) was required to assist in identifying the system or component associated with the concern.

Starting with the 1994 model year, both CARB and the Environmental Protection Agency (EPA) mandated enhanced OBD systems, commonly known as OBD-II. The objectives of the OBD-II system are to improve air quality by reducing high in-use emissions caused by emission-related concerns, reducing the time between the occurrence of a concern and its detection and repair, and assisting in the diagnosis and repair of emission-related problems.

North American OBD-II/Federal OBD requirements apply to:

• Gasoline engine passenger cars and trucks:  All California and Green State (Maine, Massachusetts, New York, Vermont) passenger cars, medium duty passenger vehicles (MDPVs), and trucks up to 6,350 kg (14,000 lb) gross vehicle weight rating (GVWR). Federal trucks from 3,856 to 6,350 kg (8,500 to 14,000 lb) GVWR complete the phasing in of OBD-II. Federal heavy-duty trucks up to 4,536 kg (10,000 lb) GVWR choosing to certify using light duty truck provisions must comply with OBD-II requirements. Federal heavy-duty trucks over 6,350 kg (14,000 lb) GVWR that do not comply with OBD-II regulations must comply with OBD-I in order to meet the minimum Ford requirements. Passenger cars and trucks sold in Canada and Mexico have Federal calibrations, unless unique calibrations are certified for Mexico at high altitude. Mexico requires OBD-II for vehicles under 3,856 kg (8,500 lb) GVWR. Trucks over 3,856 kg (8,500 lb) GVWR must comply with OBD-I in order to meet the minimum Ford requirements.
• Diesel engine passenger cars and trucks:  All California and Green State passenger cars, MDPVs, and trucks up to 6,350 kg (14,000 lb) GVWR. Federal trucks from 3,856 to 6,350 kg (8,500 to 14,000 lb) GVWR complete the phasing in of OBD-II. Federal heavy-duty trucks over 6,350 kg (14,000 lb) GVWR that do not comply with OBD-II regulations must comply with OBD-I in order to meet the minimum Ford requirements.

The OBD-II system monitors virtually all emission control systems and components that can affect tailpipe or evaporative emissions. In most cases, concerns must be detected before emissions exceed 1.5 times the applicable 100,000, 120,000, or 150,000 (passenger cars), or 120,000 (trucks) mile emission standards. Partial zero emission vehicle (PZEV) and super ultra low emission vehicle (SULEV-II) vehicles can use the concern criteria of 2.5 in lieu of the 1.5 standard whenever required. Federal Tier 2 (Bin 3 and 4) must use a 1.5 concern criteria for non-methane organic gases (NMOG) and carbon monoxide (CO), a 1.75 criteria for NMOG catalyst monitor, and a 2.5 criteria for oxides of nitrogen (NO _x ). If a system or component exceeds emission thresholds or does not operate within a manufacturer's specifications, a DTC is stored and the MIL is illuminated within 2 driving cycles.

The OBD-II system monitors for concerns either continuously, regardless of driving mode, or non-continuously, once per drive cycle during specific drive modes. A pending DTC is stored in the PCM keep alive memory (KAM) when a concern is initially detected. Pending DTCs are displayed as long as the concern is present. Note that OBD regulations required a complete concern-free monitoring cycle to occur before erasing a pending DTC. This means that a pending DTC is erased on the next power-up after a concern-free monitoring cycle. However, if the concern is still present after 2 consecutive drive cycles, the MIL is illuminated. Once the MIL is illuminated, 3 consecutive drive cycles without a concern detected are required to extinguish the MIL. The DTC is erased after 40 engine warm-up cycles once the MIL is extinguished.

In addition to specifying and standardizing much of the diagnostics and MIL operation, OBD requires the use of a standard data link connector (DLC), standard communication links and messages, standardized DTCs and terminology. Examples of standard diagnostic information are freeze frame data and inspection maintenance (IM) readiness indicators.

Freeze frame data describes data stored in the KAM at the point the concern is initially detected. Freeze frame data consists of parameters such as engine RPM and load, state of fuel control, spark, and warm-up status. Freeze frame data is stored at the time the first concern is detected; however, previously stored conditions are replaced if a fuel or misfire concern is detected. This data is accessible with the diagnostic tool to assist in repairing the vehicle.

OBD IM readiness indicators show whether all of the OBD monitors have been completed since the last time the KAM or the PCM DTC(s) have been cleared. Ford also stores a P1000 DTC to indicate that some monitors have not completed. In some states, it may be necessary to carry out an OBD check in order to renew a vehicle registration. The IM readiness indicators must show that all monitors have been completed prior to the OBD check.

Vehicles not required to comply with OBD-II requirements use an OBD-I system. OBD-I systems are used on all Federal truck calibrations over 3,856 kg (8,500 lb) GVWR. OBD-I vehicles use the same data communication link, DLC, and PCM software as the corresponding OBD-II vehicle. Differences between OBD-I and OBD-II vehicles may be removal of the rear oxygen sensor(s), fuel tank pressure sensor, canister vent solenoid, and PCM calibration. The table below lists what monitors and functions have been altered for the OBD-I calibration.

The following monitor descriptions provide a general description of each OBD monitor. In these descriptions, the monitor strategy, hardware, testing requirements, and methods are presented to provide an overall understanding of monitor operation. An illustration of each monitor may also be provided. These illustrations should be used as typical examples and are not intended to represent all possible vehicle configurations.

Each illustration depicts the PCM as the main focus with primary inputs and outputs for each monitor. The icons to the left of the PCM represent the inputs used by each of the monitor strategies to enable or activate the monitor. The components and subsystems to the right of the PCM represent the hardware and signals used while carrying out the tests and the systems being tested. The comprehensive component monitor (CCM) illustration has numerous components and signals involved which are shown generically. When referring to the illustrations, match the numbers to the corresponding numbers in the monitor descriptions for a better comprehension of the monitor and associated DTCs.

These icons are used in the illustrations of the OBD monitors and throughout this article.

Fig 1: Identifying On Board Diagnostic Monitors Icons

Courtesy of FORD MOTOR CO.