What is the onboard diagnostics system, OBDII?

OBDII is the second-generation onboard diagnostic (OBD) interface. The first generation of OBD coincided with the advent of onboard computers in vehicles. Initial OBD systems were intended to provide technicians with information on the status of various vehicle systems, with an indicator light on the dash to alert drivers of a malfunction. OBDII includes more comprehensive diagnostic trouble codes (DTCs) and emissions control.

In this article, we’ll review the basics and configurations of OBDII and related standards. We’ll also discuss how OBD has been implemented in Europe and how it’s integrated into heavy-duty trucks.

OBDII is governed by several standards, including ISO 11898, SAE J1962, J1850, J1939, J1978, J1979, J2012, and J2178-1. OBDII is also composed of several elements, including:

A comprehensive suite of sensors, engine, and exhaust controls
General vehicle functions such as monitoring power steering, battery health, airbag deployment, etc.
Dashboard interfaces for the driver
A diagnostic link connector (DLC) for use by technicians (Figure 1).

**Figure 1:** OBDII systems have several elements, including sensors, controls, diagnostics, as well as driver and technician interfaces. (Image: SMLease Design)

There are five signaling protocols currently employed by the OBD II interface through the J1962 connector, including:

1. SAE J1850 pulse width modulation (PWM) (41.6 Kbaud, Ford Motor Company Standard)

2. SAE J1850 variable pulse width (VPW) (10.4/41.6 Kbaud, General Motors Standard)

3. ISO 9141-2 (10.4 Kbaud, used in Chrysler, European, and Asian vehicles)

4. ISO 14230 KWP2000 (keyword protocol 2000) There are two variants of ISO14230-4:

5 baud init, 10.4 Kbaud
Fast init, 10.4 Kbaud

5. ISO 15765 CAN bus (250 kbps or 500 kbps) ISO15765-4 is the newest protocol. There are four variants that differ in identifier length and bus speed.

11 bit ID, 500 Kbaud
29 bit ID, 500 Kbaud
11 bit ID, 250 Kbaud
29 bit ID, 250 Kbaud

According to General Motors, VPW coding has lower EMI emissions than PWM coding. This is because VPW coding requires only one transition per transmitted bit, where as PWM requires two.

A VPW coding scheme is implemented using a Byte Data Link Controller (BDLC), which produces the data bit structure and handles all communications.

Like other OBD implementations, VPW is a single wire protocol that reduces the cost and weight of the associated wire harness. It’s an inexpensive, master-less protocol that includes arbitration, cyclic redundancy check (CRC), and acknowledgment capabilities.

ODBII originated in California to reduce automotive emissions. Today, the U.S. Environmental Protection Agency (EPA) requires all passenger cars and trucks sold in any state to meet its OBD requirements. They’re somewhat different and less strict than California’s OBDII standard.

The EPA accepts California’s requirements and, in practice, all passenger vehicles sold in the U.S. are designed and certified to meet the California OBDII requirements. As verification that OBDII is implemented, the emissions-control information label on the underside of the hood must include a specific “OBD II” notification.

The OBDII interface
OBDII uses a standardized digital communications port to provide real-time data and a standardized series DTCs. The OBDII specification requires the use of a female 16-pin (2×8) J1962 connector in the vehicle (Figure 2):

Pin 1 is for the bus-positive line of SAE J1850 PWM and VPW
Pin 4 is chassis ground
Pin 5 is signal ground
Pin 6 is CAN high for ISO 15765-4 and SAE J2284
Pin 7 is the K-line of ISO 9141-2 and ISO 14230-4
Pins 8, 9, 11, 12, and 13 are for the manufacturer’s discretion for customizations.
Pin 10 is for the bus-negative line of SAE J1850 PWM only, not used for SAE 1850 VPW
Pin 14 is CAN low for ISO 15765-4 and SAE J2284
Pin 15 is the L-line of ISO 9141-2 and ISO 14230-4
Pin 16 is for the battery voltage

**Figure 2:** OBDII’s 16-pin J1962 interface connector pin locations. (Image: Wikipedia)

In contrast to the under-the-hood location of earlier OBDI connectors, the OBDII DLC must be within two feet (0.61 m) of the steering wheel in a vehicle cabin to ensure easy access.

OBDIII
OBDIII is currently under development. It refers to the “remote OBD,” whereby future vehicles will have the ability to wirelessly send DTC alerts to remote locations such as a wireless handset, a local repair shop, or an emissions testing station.

Vehicles with OBDIII will be more capable of correcting any condition causing a DTC alert. The goal is that the owner of such a vehicle would not have to take it to a station for an emissions inspection unless it indicates active emission-related malfunctions.

With the current OBDII, the vehicle owner is under no time pressure to correct the issue if there is a DTC alert. However, with OBDIII, the owner would have a specified time limit for addressing DTCs. Remedial action and correction of the condition would be automatically forwarded to the appropriate agency. In California, this would go to the California Air Resources Board (CARB). If the correction is not made within a timely manner, a citation could be issued.

The use of OBDIII has been studied by several states, along with California, through pilot programs. Participating vehicles are retrofitted with the appropriate transmitter.

Currently, OBDIII is under consideration as a voluntary program. The motivation for vehicle owners to participate is convenience and lower inspection costs. The overall benefits include more timely, consistent, and significant emissions reductions.

EOBD and HDOBD
The European equivalent of OBDII is called EOBD and applies to all category M1 passenger cars (a gross vehicle weight of 2500 kg or less with no more than eight seats). It applies to gasoline and diesel-powered vehicles. The technical specifications of EOBD are similar toOBDII, with the same signal protocols and diagnostic connector.

In addition to OBDII and EOBD, HDOBD has been released for medium and heavy-duty trucks in the U.S. HDOBD uses different connectors than the OBDII ones for heavy-duty trucks and standard ODBII connectors (like those used in passenger cars) for light and medium-duty trucks. (Figure 3).

**Figure 3:** Heavy-duty trucks use a 6-pin (J1708) or 9-pin (J1939) connector to implement HDOBD, whereas the standard OBDII connector (shown on the far right) is used by all other vehicles. (Image: DriveELD)

Summary
OBD systems originated with the introduction of computer processors in automotive systems to monitor system performance. These systems initially monitored vehicle health monitoring and now offer comprehensive emissions-control monitoring.

The OBDII standards established in California have since been adopted throughout the U.S. There are specific ODB implementations for medium and heavy-duty trucks. In the future, ODBIII will add remote-monitoring capabilities to these systems.

References