The Powertrain Control Module (PCM) monitors for misfire during most engine operating conditions (positive torque)
by looking at changes in the crankshaft speed. If a misfire occurs, the speed of the crankshaft will vary more than
normal.
FUEL SYSTEM MONITOR
The PCM is programmed to maintain the optimum air/fuel ratio. 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 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 calculation, then 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
air/fuel ratio, then the MIL will be illuminated.
CATALYST MONITOR
DESCRIPTION - To comply with clean air regulations, vehicles are equipped with catalytic converters. These con-
verters reduce the emission of hydrocarbons, oxides of nitrogen and carbon monoxide.
Normal vehicle miles or engine misfire can cause a catalyst to decay. A meltdown of the ceramic core can cause a
restriction of the exhaust. This can increase vehicle emissions and deteriorate engine performance, driveability and
fuel economy.
The catalyst monitor uses dual oxygen sensors (O2Ss) to monitor the efficiency of the converter. The dual O2S
strategy is based on the fact that as a catalyst deteriorates, its oxygen storage capacity and its efficiency are both
reduced. By monitoring the oxygen 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 con-
verter. The PCM calculates the air/fuel 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 high oxygen 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 the down-
stream O2S. When the efficiency drops, no chemical reaction takes place. This means the concentration of oxygen
will be the same downstream as upstream. The output voltage of the downstream O2S copies the voltage of the
upstream sensor. The only difference is a time lag (seen by the PCM) between the switching of the O2Ss.
To monitor the system, the number of lean-to-rich switches of upstream and downstream O2Ss is counted. The ratio
of downstream switches to upstream switches is used to determine whether the catalyst is operating properly. An
effective catalyst will have fewer downstream switches than it has upstream switches i.e., a ratio closer to zero. For
a totally ineffective catalyst, this ratio will be one-to-one, indicating that no oxidation occurs in the device.
The system must be monitored so that when catalyst efficiency deteriorates and exhaust emissions increase to over
the legal limit, the MIL will be illuminated.
OPERATION - To monitor catalyst efficiency, the PCM expands the rich and lean switch points of the heated oxygen
sensor. 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 O2S 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 O2S switches is divided by the number of Front O2S switches to determine the switching ratio.
The test runs for 20 seconds. As catalyst efficiency deteriorates over the life of the vehicle, the switch rate at the
downstream sensor approaches that of the upstream sensor. If at any point during the test period the switch ratio
reaches a predetermined value, a counter is incremented by one. The monitor is enabled to run another test during
that trip. When the test fails three times, the counter increments to three, a malfunction is entered, and a Freeze
Frame is stored. When the counter increments to three during the next trip, the code is matured and the MIL is
illuminated. If the test passes the first, no further testing is conducted during that trip.
The MIL is extinguished after three consecutive good trips.
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EMISSIONS CONTROL
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