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 temperature 300° to 350°C (572 ° to 662°F), the
sensor generates a voltage that is inversely propor-
tional to the amount of oxygen in the exhaust. The
information obtained by the sensor is used to calcu-
late the fuel injector pulse width. This maintains 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 300°C. 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.
HEX 2E—EGR MONITOR
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
O2S 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.
HEX 6A,6B, 6C, 6D, 6E, AE, and AF—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.
HEX 76, 77, 78, and 79—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 of 14.7 to 1. This is done by making
short term corrections in the fuel injector pulse width
based on the O2S output. The programmed memory
acts as a self calibration tool that the engine control-
ler uses to compensate for variations in engine spec-
ifications, 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 multiply-
ing that with the program long-term (adaptive) mem-
ory and comparing that to the limit, it can be
determined whether it will pass an emissions test. If
a malfunction occurs such that the PCM cannot
maintain the optimum A/F ratio, then the MIL will
be illuminated.
HEX 70, and B4—CATALYST 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.
Normal vehicle miles or engine misfire can cause a
catalyst to decay. A meltdown of the ceramic core can
cause a reduction of the exhaust passage. This can
increase vehicle emissions and deteriorate engine
performance, driveability and fuel economy.
The catalyst monitor uses dual oxygen sensors
(O2S’s) to monitor the efficiency of the converter. The
dual O2S’s strategy is based on the fact that as a cat-
alyst deteriorates, its oxygen storage capacity and its
efficiency are both reduced. By monitoring the oxy-
gen storage capacity of a catalyst, its efficiency can
be indirectly calculated. The upstream O2S is used to
detect the amount of oxygen in the exhaust gas
before the gas enters the catalytic converter. The
PCM calculates the A/F mixture from the output of
the O2S. A low voltage indicates high oxygen content
(lean mixture). A high voltage indicates a low content
of oxygen (rich mixture).
When the upstream O2S detects a lean condition,
there is an abundance of oxygen in the exhaust gas.
A functioning converter would store this oxygen so it
can use it for the oxidation of HC and CO. As the
converter absorbs the oxygen, there will be a lack of
oxygen downstream of the converter. The output of
the downstream O2S will indicate limited activity in
this condition.
As the converter loses the ability to store oxygen,
the condition can be detected from the behavior of
300M
EMISSION CONTROL SYSTEMS
25 - 9
DESCRIPTION AND OPERATION (Continued)