The information obtained by the sensor is used to
calculate the fuel injector pulse width. The PCM is
programmed to maintain the optimum air/fuel 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 (if equipped), Catalyst and Fuel Monitors.
The O2S may fail in any or all of the following
manners:
²Slow response rate
²Reduced output voltage
²Dynamic shift
²Shorted or open circuits
Response rate is the time required for the sensor to
switch from lean to rich once it is exposed to a richer
than optimum A/F mixture or vice versa. As the sen-
sor starts malfunctioning, it could take longer to
detect the changes in the oxygen content of the
exhaust gas.
The output voltage of the O2S ranges from 0 to 1
volt (voltages are offset by 2.5 volts on NGC vehi-
cles). A good sensor can easily generate any output
voltage in this range as it is exposed to different con-
centrations 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.
OXYGEN SENSOR HEATER MONITOR
If there is an oxygen sensor (O2S) DTC as well as
a O2S heater DTC, the O2S heater fault MUST be
repaired first. After the O2S fault is repaired, verify
that the heater circuit is operating correctly.
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 oper-
ating temperatures of 300É to 350ÉC (572 É to 662ÉF),
the sensor generates a voltage that is inversely pro-
portional to the amount of oxygen in the exhaust.
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 nitrogen oxide
(NOx) from the exhaust.
The voltage readings taken from the O2S are very
temperature sensitive. The readings are not accurate
below 300ÉC. Heating of the O2S is done to allow the
engine controller to shift to closed loop control as
soon as possible. The heating element used to heat
the O2S must be tested to ensure that it is heating
the sensor properly.The O2S circuit is monitored for a drop in voltage.
The sensor output is used to test the heater by iso-
lating the effect of the heater element on the O2S
output voltage from the other effects.
EGR MONITOR (if equipped)
The Powertrain Control Module (PCM) performs
an on-board diagnostic check of the EGR system.
The EGR monitor is used to test whether the EGR
system is operating within specifications. The diag-
nostic check activates only during selected engine/
driving conditions. When the conditions are met, the
EGR is turned off (solenoid energized) and the O2S
compensation control is monitored. Turning off the
EGR shifts the air fuel (A/F) ratio in the lean direc-
tion. The O2S data should indicate an increase in the
O2 concentration in the combustion chamber when
the exhaust gases are no longer recirculated. While
this test does not directly measure the operation of
the EGR system, it can be inferred from the shift in
the O2S data whether the EGR system is operating
correctly. Because the O2S is being used, the O2S
test must pass its test before the EGR test. Also
looks at EGR linear potentiometer for feedback.
MISFIRE MONITOR
Excessive engine misfire results in increased cata-
lyst temperature and causes an increase in HC emis-
sions. Severe misfires could cause catalyst damage.
To prevent catalytic convertor damage, the PCM
monitors engine misfire.
The Powertrain Control Module (PCM) monitors
for misfire during most engine operating conditions
(positive torque) by looking at changes in the crank-
shaft speed. If a misfire occurs the speed of the
crankshaft will vary more than normal.
FUEL SYSTEM MONITOR
To comply with clean air regulations, vehicles are
equipped with catalytic converters. These converters
reduce the emission of hydrocarbons, oxides of nitro-
gen and carbon monoxide. The catalyst works best
when the air fuel (A/F) ratio is at or near the opti-
mum of 14.7 to 1.
The PCM is programmed to maintain the optimum
air/fuel ratio. This is done by making short term cor-
rections in the fuel injector pulse width based on the
O2S output. The programmed memory acts as a self
calibration tool that the engine controller uses to
compensate for variations in engine specifications,
sensor tolerances and engine fatigue over the life
span of the engine. By monitoring the actual air-fuel
ratio with the O2S (short term) and multiplying that
with the program long-term (adaptive) memory and
comparing that to the limit, it can be determined
whether it will pass an emissions test. If a malfunc-
RSEMISSIONS CONTROL25-7
EMISSIONS CONTROL (Continued)
