138 Auto Engine Performance and Driveability
On-Board Diagnostics Generation
One (OBD I)
Some form of on-board diagnostics has always been
used in computer-controlled vehicles. These early self-
diagnostic systems are sometimes referred to as on-board
diagnostics generation one (OBD I). The self-diagnostic
system not only checks the ECM for proper operation, but
monitors the sensors and output devices to ensure that there
are no problems. If a sensor or device becomes defective,
is disconnected, or provides readings that are out-of-range
due to a non-computer-related problem, the ECM stores a
diagnostic trouble code (DTC). On most OBD I systems,
this code is a two or three digit number that corresponds
to a certain malfunction condition. The ECM illuminates
the dash-mounted malfunction indicator light (MIL) when
a code is present.
On-Board Diagnostics Generation
Two (OBD II)
All 1996 and newer vehicles sold in the United States
are equipped with OBD II. On-board diagnostics genera-
tion two (OBD II) not only detects sensor malfunctions and
driveability problems when they occur, but also detects
potential problems before they affect emissions or even
become noticeable to the driver.
Some manufacturers may use other names for their
version of the OBD II system. Basic operating principles
are the same for all systems, no matter what they are called.
OBD II is the term used by the Society of Automotive
Engineers (SAE) for enhanced diagnostics systems. This
textbook refers to all such systems as OBD II.
Vehicles equipped with OBD II are similar in many
ways to other vehicles with older diagnostic systems.
However, several additional parameters must be met:
p
Redundant sensors for each monitored system. For
example, many OBD II vehicles use both a MAF sen-
sor and a MAP sensor.
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Most OBD II vehicles have multiple, heated oxygen
sensors before the converter.
p
All OBD II vehicles have a heated oxygen sensor after
the converter (catalyst monitor).
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Most EGR valves are electronically operated and
equipped with a pintle position sensor.
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Sequential multiport fuel injection is used on almost
all engines.
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Evaporative emissions systems are equipped with diag-
nostic switches for purge monitoring. More advanced
systems include test fittings, vent solenoids, and a fuel
tank–pressure sensor.
p
All OBD II equipped vehicles must monitor the air
conditioning system for low refrigerant levels, which
indicates a leak.
Some service literature for OBD II vehicles has an
emblem to indicate the vehicle is equipped with OBD II. The
emblem can be found on schematic pages and other charts.
OBD II ECM
The ECM in an OBD II system uses Class 2 data com-
munication. In this communication, data are transmitted at
a variable rate of 10.4 to 41.6 kilobits per second (kbps).
The ECM in an OBD II system also runs monitors. Monitors
are self checks of various internal and external systems and
components. The ECM runs monitors under various engine
and atmospheric conditions. In some climates or under
some operating conditions, the ECM may not run a particu-
lar monitor. If the monitoring process detects a problem,
the ECM sets a trouble code.
OBD II PROMs
The OBD II protocol mandates that all PROMs be
permanently affixed to the ECM. One of the reasons for
this is to prevent installation of aftermarket “hot PROMs.”
These are designed to enhance a computer-controlled
vehicle’s performance, which often increases exhaust emis-
sions. Most newer ECMs use EEPROMs or FEPROMs that
are write-protected and can only be reprogrammed using
special equipment.
Figure 7-36. OBD II data link connectors are located within easy
access of the driver’s seat. (Chrysler)
Instrument panel
Diagnostic
connector
Instrument
cluster
“Check engine”
malfunction indicator light (MIL)