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
this monitoring circuit. As the temperature increases,
resistance decreases and the PCM detects a lower
voltage at the reference signal. Inversely, as the tem-
perature decreases, the resistance increases and the
PCM detects a higher voltage at the reference signal.
an The O2S circuit is monitored for a drop in voltage.
OPERATION— 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,
the
internal
resistance
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
25 - 18
EMISSION CONTROL SYSTEMS
NS
DESCRIPTION AND OPERATION (Continued)
