onboardd.pdf

Section 2

On-Board Diagnostic Systems

Learning Objectives:

1.Determine status of OBD II controlled systems based on MIL status.

2.Determine status of OBD II Readiness Tests Monitors using

CARB mode.

3.Interpret OBD II SAE Powertrain DTC nomenclature.

4.Identify OBD II Scan Tool modes and apply these modes to a

diagnostic routine.

Engine Control Systems II - Course 874

Section 2

On-Board Diagnostic Systems

Overview

On-Board Diagnostic (OBD) systems use the vehicle’s computer(s) to

detect problems with components or systems and report these problems

to driver and technician. Engine OBD systems are, in part, governed by

regulations and divided into two major categories:

• OBD I

• OBD II (phased in beginning 1996 MY)

In 1988, the California Air Resources Board (CARB) set the requirement

that all vehicles have a system that could identify faults in the emission

and powertrain system. This is recognized as OBD I.

At the same time, CARB also set the requirements for OBD II. The

Federal government adopted these requirements and they went into

effect beginning in 1996. OBD II standards greatly enhanced the On-

Board diagnostic system’s capabilities and changed the way technicians

troubleshoot engine and emission control systems.

In either case, the manufacturer can provide additional diagnostic

capabilities. For the technician, understanding what the OBD system is

capable of and its limitations will help in fixing vehicles right the first

time.

OBD systems report data to the technician by a Malfunction Indicator

Lamp (MIL) located in the instrument cluster and Diagnostic Tester.

Engine Control Systems II - Course 874 2-1

Section 2

OBD I Regulations

• Current related checks (open or short)

• Limited system monitoring (A/F & EGR)

• Minimal use of rationality checks

• Limited DTC(s)

• Limited use of Serial data

• System and component names not standardized

• DTC(s) not standardized

Fig. 2-1

TL874f201

OBDI(On-

Board

Diagnostic

System,

(Generation 1)

In April 1985, the California Air Resources Board (CARB) approved On-

Board Diagnostic system regulations, referred to as OBD. Beginning in

1988, these regulations were phased in to include cars and light trucks

marketed in the State of California. They required that the ECM monitor

critical emission related components for proper operation and illuminate

a malfunction indicator lamp (MIL) on the instrument panel when a

malfunction was detected.

Although the OBD regulations initially apply to California emissions

certified vehicles, some or all of the OBD system features are found on

Federal emissions certified vehicles as well.

The OBD system uses Diagnostic Trouble Codes (DTC) and fault isolation

logic charts in the Repair Manual, to assist technicians in determining

the likely cause of engine control and emissions system malfunctions.

The basic objectives of this regulation are:

• To improve in-use emissions compliance by alerting the vehicle

operator when a malfunction exists.

• To aid repair technicians in identifying and repairing malfunctioning

circuits in the emissions control system.

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TOYOTA Technical Training

On-Board Diagnostic Systems

OBD applies to systems that are considered most likely to cause a

significant increase in exhaust emissions when a malfunction occurs.

Commonly, this includes:

• All major engine sensors

• The fuel metering system

• Exhaust Gas Recirculation (EGR) function

Components and circuits are monitored for continuity, shorts, and in

some cases, normal parameter range. OBD systems were normally

limited to the detection of an open or short in a sensor circuit.

OBD Malfunction

Indicator Lamp

(MIL)

The MIL is required to serve as a visual alert to the driver of a

malfunction in the system. When a malfunction occurs, the MIL

remains illuminated as long as the fault is detected and goes off once

normal conditions return, leaving a Diagnostic Trouble Code (DTC)

stored in the ECM memory.

OBD Diagnostic

Trouble Codes

(DTC)

DTC(s) are generated by the on-board diagnostic system and stored in

the ECM memory. They indicate the circuit in which a fault has been

detected. DTC information remains stored in the ECM long-term

memory regardless of whether a continuous (hard) fault or intermittent

fault caused the code to set. OBD vehicles store a DTC in the ECM long-

term memory until power is removed from the ECM. In most cases, the

EFI fuse powers this long-term (keep alive) memory.

Engine Control Systems II - Course 874

2-3

Section 2

OBD II

• Circuit continuity and out of range values

monitored

• System monitored

• Rationality checks used (logic)

• Expanded DTC(s)

• Freeze Frame Data stored with DTC

• Serial Data required

• Active Tests

• Standards established

Fig. 2-2

TL874f202

ODBII(On-

Board

Diagnostic

System,

Generation 2)

OBD II requires the ECM to monitor the effectiveness of the major

emission control systems and to turn on the MIL when a malfunction is

detected or when the performance of the emission system(s) has

deteriorated to where the emission output will exceed the allowed

emission levels.

All vehicles sold in the United States are certified through the Federal

Test Procedure (FTP). It is the FTP that tests and sets maximum

emission levels in accordance with government regulations. The MIL

must light when a component or system will cause the vehicle’s

emission levels to exceed 1-1/2 times the FTP standard. This means

that the OBD II system must test the performance of a system or

component. For example, the ECM OBD system monitors catalytic

converter efficiency. If catalytic converter efficiency is out of range, the

MIL will illuminate and a DTC will set.

OBD II

Standardization

OBD regulations and technical standards have been developed with the

cooperation of the automotive industry and the Society of Automotive

Engineers (SAE). These standards provide a common format for data,

the Diagnostic Tester, diagnostic test modes, and diagnostic trouble

codes regardless of the vehicle manufacturer.

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