emissions-related components for electrical opens and
shorts.
However, OBD II also requires that inputs from
powertrain components to the PCM be tested for
rationality, and that outputs to powertrain compo-
nents from the PCM be tested for functionality.
Methods for monitoring the various Comprehensive
Component monitoring include:
(1) Circuit Continuity
•
Open
•
Shorted high
•
Shorted to ground
(2) Rationality or Proper Functioning
•
Inputs tested for rationality
•
Outputs tested for functionality
NOTE: Comprehensive
component
monitors
are
continuous. Therefore, enabling conditions do not
apply.
Input Rationality— While input signals to the
PCM are constantly being monitored for electrical
opens and shorts, they are also tested for rationality.
This means that the input signal is compared against
other inputs and information to see if it makes sense
under the current conditions.
PCM sensor inputs that are checked for rationality
include:
•
Manifold Absolute Pressure (MAP) Sensor
•
Oxygen Sensor (O2S)
•
Engine Coolant Temperature (ECT) Sensor
•
Camshaft Position (CMP) Sensor
•
Vehicle Speed Sensor
•
Crankshaft Position (CKP) Sensor
•
Intake Air Temperature (IAT) Sensor
•
Throttle Position (TPS) Sensor
•
Ambient/Battery Temperature Sensors
•
Power Steering Switch
•
Oxygen Sensor Heater
•
Engine Controller
•
Brake Switch
•
Leak Detection Pump Switch
•
P/N Switch
•
Trans Controls
Output Functionality— PCM outputs are tested
for functionality in addition to testing for opens and
shorts. When the PCM provides a voltage to an out-
put component, it can verify that the command was
carried out by monitoring specific input signals for
expected changes. For example, when the PCM com-
mands the Idle Air Control (IAC) Motor to a specific
position under certain operating conditions, it expects
to see a specific (target) idle speed (RPM). If it does
not, it stores a DTC.
PCM outputs monitored for functionality include:
•
Fuel Injectors
•
Ignition Coils
•
Torque Converter Clutch Solenoid
•
Idle Air Control
•
Purge Solenoid
•
EGR Solenoid
•
LDP Solenoid
•
Radiator Fan Control
•
Trans Controls
OXYGEN SENSOR (O2S) MONITOR
DESCRIPTION— Effective control of exhaust
emissions is achieved by an oxygen feedback system.
The most important element of the feedback system
is the O2S. The O2S is located in the exhaust path.
Once it reaches operating temperature 300° to 350°C
(572° to 662°F), the sensor generates a voltage that
is inversely proportional to the amount of oxygen in
the exhaust. When there is a large amount of oxygen
in the exhaust caused by a lean condition, the sensor
produces a low voltage, below 450 mV. When the oxy-
gen content is lower, caused by a rich condition, the
sensor produces a higher voltage, above 450mV.
The information obtained by the sensor is used to
calculate the fuel injector pulse width. This main-
tains a 14.7 to 1 air fuel (A/F) ratio. At this mixture
ratio, the catalyst works best to remove hydrocarbons
(HC), carbon monoxide (CO) and nitrous oxide (NOx)
from the exhaust.
The O2S is also the main sensing element for the
EGR, Catalyst and Fuel Monitors.
The O2S may fail in any or all of the following
manners:
•
Slow response rate (Big Slope)
•
Reduced output voltage (Half Cycle)
•
Heater Performance
Slow Response Rate (Big Slope)— Response
rate is the time required for the sensor to switch
from lean to rich signal output once it is exposed to a
richer than optimum A/F mixture or vice versa. As
the PCM adjusts the air/fuel ratio, the sensor must
be able to rapidly detect the change. As the sensor
ages, it could take longer to detect the changes in the
oxygen content of the exhaust gas. The rate of
change that an oxygen sensor experiences is called
’Big Slope’. The PCM checks the oxygen sensor volt-
age in increments of a few milliseconds.
Reduced Output Voltage (Half Cycle)— The
output voltage of the O2S ranges from 0 to 1 volt. A
good sensor can easily generate any output voltage in
this range as it is exposed to different concentrations
of oxygen. To detect a shift in the A/F mixture (lean
or rich), the output voltage has to change beyond a
threshold value. A malfunctioning sensor could have
difficulty changing beyond the threshold value. Each
time the voltage signal surpasses the threshold, a
counter is incremented by one. This is called the Half
Cycle Counter.
NS
EMISSION CONTROL SYSTEMS
25 - 17
DESCRIPTION AND OPERATION (Continued)
