• Heater Performance
Slow Response Rate (Big Slope)— Response
rate is the time required for the sensor to switch
from lean to rich signal output once it is exposed to a
richer than optimum A/F mixture or vice versa. As
the PCM adjusts the air/fuel ratio, the sensor must
be able to rapidly detect the change. As the sensor
ages, it could take longer to detect the changes in the
oxygen content of the exhaust gas. The rate of
change that an oxygen sensor experiences is called
’Big Slope’. The PCM checks the oxygen sensor volt-
age in increments of a few milliseconds.
Reduced Output Voltage (Half Cycle)— The
output voltage of the O2S ranges from 0 to 1 volt. A
good sensor can easily generate any output voltage in
this range as it is exposed to different concentrations
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. Each
time the voltage signal surpasses the threshold, a
counter is incremented by one. This is called the Half
Cycle Counter.
Heater Performance— The heater is tested by a
separate monitor. Refer to the Oxygen Sensor Heater
Monitor.
OPERATION— As the Oxygen Sensor signal
switches, the PCM monitors the half cycle and big
slope signals from the oxygen sensor. If during the
test neither counter reaches a predetermined value, a
malfunction is entered and a Freeze Frame is stored.
Only one counter reaching its predetermined value is
needed for the monitor to pass.
The Oxygen Sensor Monitor is a two trip monitor
that is tested only once per trip. When the Oxygen
Sensor fails the test in two consecutive trips, the
MIL is illuminated and a DTC is set. The MIL is
extinguished when the Oxygen Sensor monitor
passes in three consecutive trips. The DTC is erased
from memory after 40 consecutive warm-up cycles
without test failure.
Enabling Conditions— The following conditions
must typically be met for the PCM to run the oxygen
sensor monitor:
• Battery voltage
• Engine temperature
• Engine run time
• Engine run time at a predetermined speed
• Engine run time at a predetermined speed and
throttle opening
• Transmission in gear (automatic only)
• Fuel system in Closed Loop
• Long Term Adaptive (within parameters)
• Power Steering Switch in low PSI (no load)
• Engine at idle
• Fuel level above 15%
• Ambient air temperature
• Barometric pressure
• Engine RPM within acceptable range of desired
idle
• Closed throttle speed
Pending Conditions— The Task Manager typi-
cally does not run the Oxygen Sensor Monitor if over-
lapping monitors are running or the MIL is
illuminated for any of the following:
• Misfire Monitor
• Front Oxygen Sensor and Heater Monitor
• MAP Sensor
• Vehicle Speed Sensor
• Engine Coolant Temperature Sensor
• Throttle Position Sensor
• Engine Controller Self Test Faults
• Cam or Crank Sensor
• Injector and Coil
• Idle Air Control Motor
• EVAP Electrical
• EGR Solenoid Electrical
• Intake Air Temperature
• 5 Volt Feed
Conflict— The Task Manager does not run the
Oxygen Sensor Monitor if any of the following condi-
tions are present:
• A/C ON (A/C clutch cycling temporarily sus-
pends monitor)
• Purge flow in progress
Suspend— The Task Manager suspends maturing
a fault for the Oxygen Sensor Monitor if an of the fol-
lowing are present:
• Oxygen Sensor Heater Monitor, Priority 1
• Misfire Monitor, Priority 2
OXYGEN SENSOR HEATER MONITOR
DESCRIPTION— If there is an oxygen sensor
(O2S) DTC as well as a O2S heater DTC, the O2S
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. The sensor
output is used to test the heater by isolating the
effect of the heater element on the O2S output volt-
age from the other effects. The resistance is normally
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
25 - 16 EMISSION CONTROL SYSTEMS LH
DESCRIPTION AND OPERATION (Continued)
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