ENGINE <NON-TURBO>
-
Control System
02
Heater Monitor
L
Background
In order for the
02
sensor to function properly, the
02
sensor must be heated to approximately 572“
-
662°F. To assist the
02
sensor in achieving this
temperature, the
O2
sensor is equipped with a Posi-
tive Thermal Coefficient (PTC) heater element. Both
the upstream an downstream’s heater element is
fed battery voltage any time the MFI relay is ener-
gized. The ground for the heater element is routed
through the
02
sensor’s
4-pin
connector to an exter-
nal ground source. As current flows through the
element, the temperature of the
O2
sensor in-
creases. As the temperature increases, the resis-
tance of the element increases causing the current
Operation
The resistance of the
02
sensor changes with tem-
perature and age. It’s resistance normally is between
100
Sz
to 4.5
MegR.
When the temperature of the
sensor is increased, the resistance of the sensor
decreases. Inversely, as the sensor ages, the resis-
tance increases. It is the resistance of the sensor’s
output circuit that is tested for proper heater opera-
tion, not the heater element itself.
L
The test begins approximately 5 seconds after the
engine has been turned off with the ignition key
and battery voltage greater than 10 volts (the PCM
still operates even though the key is in the off posi-
tion). Once the timer has timed out, the PCM bias
5 volts to the
O2
sensors output wire once every
1.6 seconds, and keeps it biased each time for
35 ms. During this portion of the test, the PCM
monitors the voltage on the output wire of the
02
sensor. As the sensor cools down, the resistance
should increase, causing the PCM to register an
increase of voltage. The PCM determines a
02
sen-
flow to decrease. The
02
sensor’s heater maintains
the temperature of the sensor to around 1200°F.
After the engine has started, the upstream
02
sen-
sor’s information is used by the PCM to assist in
the air/fuel calculation. If the sensor is not up to
operating temperatures, the information given by
the sensor may be inaccurate, possibly causing
an increase in emissions.
The downstream
O2
sensor is used by the PCM
to calculate the efficiency of the catalytic converter.
The sensor must be heated to allow the 02 sensor
to function normally. Without the
O2
functioning nor-
mally, the catalytic converter test would be invalid.
sor that has cooled enough by detecting an increase
of 0.49
-
1.56 volts higher what the PCM detected
at the beginning of the test. The maximum amount
of time to perform this portion of the test is 144
seconds.
When the
O2
sensor has cooled enough, the PCM
energizes the MFI relay for the next 48 seconds.
With the relay energized, current should flow through
the heater element causing an increase of tempera-
ture at the sensor. As the heater warms the 02
sensor, resistance of the 02 sensors output circuit
should decrease. As the resistance decreases, volt-
age at the PCM decreases. While the MFI relay
is energized, the PCM pulses the 5 volt biased signal
30 times. Each time the biased voltage is activated,
the PCM checks for a voltage drop. The 02 heater
monitor test passes if the PCM detects at least
0.157 volt decrease in 15 out of the 30 pulsed sig-
nals.
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