Chapter
D
: Emission Control Systems Diagnosis and Repair
80
The oxygen content in a catalyst is important for efficient
conversion of exhaust gases. When a lean airfuel ratio
is present for an extended period, oxygen content in a
catalyst can reach a maximum. When a rich air-fuel ratio
is present for an extended period, the oxygen content in
the catalyst can become totally depleted. When this oc-
curs, the catalyst fails to convert the gases. This is
known as catalyst "punch through."
Catalyst operation is dependent on its ability to store and
release the oxygen needed to complete the emissions-
reducing chemical reactions. As a catalyst deteriorates,
its ability to store oxygen is reduced. Since the catalyst's
ability to store oxygen is somewhat related to proper op-
eration, oxygen storage can be used as an indicator of
catalyst performance. To accomplish this, two oxygen
sensors are required, figure 4-13.
By utilizing an oxygen sensor upstream from the catalyt-
ic converter, and a second sensor located downstream,
oxygen storage can be determined by comparing the two
voltage signals.
As the PCM constantly adjusts the air-fuel mixture to
maintain a stoichiometric ratio, the change in oxygen
content is less noticeable downstream from the convert-
er. The upstream sensor switches voltage more rapidly
than the downstream sensor. Due to the effects of the
converter, the oxygen content at the downstream sensor
should be stable. Hence the voltage output from the
downstream sensor changes less rapidly.
To monitor catalyst efficiency, the PCM expands the rich
and lean switch points of the H02S. With extended
switch points, the air-fuel mixture runs richer and leaner
to overburden the catalytic converter. Once the test is
started, the air-fuel mixture runs rich and lean and the
H02S switches are counted. A switch is counted when
an oxygen sensor signal goes from below the lean
threshold to above the rich threshold. The number of
rear sensor switches is divided by the number of front
sensor switches to determine the switching ratio.
As catalyst efficiency deteriorates over the life of the ve-
hicle, the switch rate at the downstream sensor ap-
proaches that of the upstream sensor. If at any point dur-
ing the test period the switch ratio reaches a
predetermined value, a DTC may be set and the MIL is
illuminated. If the test passes the first time, no further
testing is conducted during that trip.
There are several factors that can adversely affect the
monitoring of catalyst efficiency. They are:
• Exhaust leaks. Allowing excess amounts of oxygen
to enter the exhaust system can mask a faulty con-
verter
• Fuel contaminants such as engine oil, coolant, phos-
phorus, lead, silica, and sulfur can interfere with the
converter's chemical reaction, affecting the catalyst's
oxygen storage capacity
EVAPORATIVE EMISSION CONTROLS
The evaporative (EVAP) control system prevents fuel
tank vapors from entering the atmosphere. Fuel evapo-
ration emits hydrocarbons (HC) directly into the atmos-
phere. As fuel evaporates in the fuel tank, vapors are
routed into a charcoal canister. Through the use of a
purge solenoid, manifold vacuum draws these vapors
into the combustion chamber.
• OBD II regulations require that the diagnostic
system:
• Verify airflow from the canister to the engine
• Monitor for HC loss to the atmosphere
• Regulations also require that "the MIL shall illumi-
nate and a fault code shall be stored. Manufacturers
may employ a second warning indicator (other than
the MIL) for a leak caused by a missing or loose fuel
cap
Fig. 4-12.
Three-way converter operation.
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