Chrysler Sebring 2005 Service Manual page 2159

25 - 4 EMISSIONS CONTROL
EMISSIONS CONTROL (Continued)
OXYGEN SENSOR HEATER MONITOR (SBEC)
D ES CRIP TION —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.
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 heater element itself is not tested directly. The
sensor output is used to test the heater by isolating
the effect of the heater element on the O2S output
voltage from the other effects. The resistance is nor-
mally between 100 ohms and 4.5 megaohms. When
oxygen sensor temperature increases, the resistance
in the internal circuit decreases. The PCM sends a 5
volts biased signal through the oxygen sensors to
ground this monitoring circuit. As the temperature
increases, resistance decreases and the PCM detects
a lower voltage at the reference signal. Inversely, as
the temperature decreases, the resistance increases
and the PCM detects a higher voltage at the refer-
ence signal. The O2S circuit is monitored for a drop
in voltage.
OP ERATION —The Oxygen Sensor Heater Moni-
tor begins after the ignition has been turned OFF
and the O2 sensors have cooled. The PCM sends a 5
volt bias to the oxygen sensor every 1.6 seconds. The
PCM keeps it biased for 35 ms each time. As the sen-
sor cools down, the resistance increases and the PCM
reads the increase in voltage. Once voltage has
increased to a predetermined amount, higher than
when the test started, the oxygen sensor is cool
enough to test heater operation.
When the oxygen sensor is cool enough, the PCM
energizes the ASD relay. Voltage to the O2 sensor
begins to increase the temperature. As the sensor
temperature increases,
decreases. The PCM continues biasing the 5 volt sig-
nal to the sensor. Each time the signal is biased, the
PCM reads a voltage decrease. When the PCM
detects a voltage decrease of a predetermined value
for several biased pulses, the test passes.
The heater elements are tested each time the
engine is turned OFF if all the enabling conditions
are met. If the monitor fails, the PCM stores a
maturing fault and a Freeze Frame is entered. If two
consecutive tests fail, a DTC is stored. Because the
ignition is OFF, the MIL is illuminated at the begin-
ning of the next key cycle, after the 2nd failure.
the internal resistance
En a blin g Co n d itio n s —The following conditions
must be met for the PCM to run the oxygen sensor
heater test:
• Engine run time of at least 5.1 minutes
• Key OFF power down
• Battery voltage of at least 10 volts
• Sufficient Oxygen Sensor cool down
P e n d in g Co n d itio n s —There are not conditions or
situations that prompt conflict or suspension of test-
ing. The oxygen sensor heater test is not run pending
resolution of MIL illumination due to oxygen sensor
failure.
S u s p e n d —There are no conditions which exist for
suspending the Heater Monitor.
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 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 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 downstream O2S. When the efficiency drops, no
chemical reaction takes place. This means the con-
centration 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 O2S's.
JR