Chapter A: General Diagnosis
12
signal that is outside its limits, it records a diagnostic
trouble code (DTC) in its memory.
Generally, the computer is capable of recognizing the
following:
• A particular signal, such as engine speed, is not
being furnished
• A signal that is out of limits for too long, such as a
too-rich or too-lean oxygen sensor (HO2S) signal
• An improbable signal, such as input from a baro-
metric pressure sensor that indicates the vehicle is
being driven at an altitude of 25,000 feet
Some systems have the ability to test sensor or actua-
tor circuit continuity by sending out a test signal and
monitoring the voltage of the return signal.
When an out-of-range signal is detected, the computer
records it as either a continuous or intermittent fault. A
continuous, or hard, failure indicates that the malfunc-
tion occurred and is still present. These generally result
from the total failure of a component or subsystem. An
intermittent, or soft, failure indicates that the malfunc-
tion took place momentarily, then disappeared. This
type of code usually means that a component or sub-
system is functioning erratically, and is often caused by
a loose, dirty, or weak connection. Most systems will
store an intermittent DTC in the long-term memory for
50 to 60 engine-start cycles.
Retrieving Codes
Most vehicles have a diagnostic connector, or data link
connector (DLC), for accessing the computer memory.
On older model vehicles, the multi-plug connector can
be either under the hood or in the passenger compart-
ment. The DLC is always in the passenger compartment
on late-model, Onboard Diagnostic II (OBD-II)-compli-
ant vehicles. The DLC permits connection of a test
meter or jumper wire to trigger the diagnostic mode. A
DTC is displayed generally in one of four basic ways:
• Numerical display on a scan tool
• Pulsating voltmeter needle
• Pulsating instrument panel lamp
• On a digital instrument cluster display panel
The manner in which the program is activated, and the
sequence in which codes are displayed varies between
manufacturers. The preferred method of DTC retrieval is
with a scan tool, figure 1-12. However, not all systems
transmit codes to a scan tool, and codes must be read
some other way. Each manufacturer publishes a list of
trouble codes used with their systems and specific in-
structions for retrieving codes and clearing the comput-
er memory.
Interpreting Diagnostic Trouble Codes
When properly used, a DTC helps to organize an effi-
cient approach to isolating the source of a problem. Al-
though trouble codes are a valuable diagnostic aid, they
will not reveal the precise cause of a problem. Howev-
er, a DTC does indicate the particular circuit where a
malfunction took place. An accurate troubleshooting
chart from the manufacturer is required to determine
what may have set the code.
Stored codes are often the result of poor circuit con-
nections rather than a component failure. Engine sensor
systems operate on low voltage, usually 5.0 volts, and
relatively minor changes in resistance will interfere with
the signal. Stored codes may be the result of a me-
chanical problem, and additional engine tests are re-
quired to isolate the source. Always check the circuitry
and connections before replacing any electronic parts.
Simply disconnecting and reconnecting a multi-plug will
often clean the contacts, restore continuity, and elimi-
nate the code. When multiple codes are present, look
for a weak connection at a common grounding point. An
open circuit results in either a signal not being generat-
ed or received. Low-voltage signals can indicate a
shorted circuit.
Individual circuits, sensors, and actuators can be
checked by connecting a breakout box (BOB) to the
computer, figure 1-13. The breakout box allows you to
monitor the individual circuit signals with a digital multi-
meter (DMM) or oscilloscope. Procedures for testing
components are discussed in the appropriate chapters
of this book.
Exhaust Gas Analyzers
To properly diagnose fuel system concerns an exhaust
gas analyzer should be used, figure 1-14. The analyzers
Fig. 1-12.
Retrieving codes with a scan tool.
Summary of Contents for ASE-A8
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