130 Auto Engine Performance and Driveability
Copyright by Goodheart-Willcox Co., Inc.
Oxygen Sensors
Using a chemical reaction to produce an electric cur-
rent is a common process. This is the source of electricity in
all batteries. The reaction between various chemicals pro-
duces extra electrons, which creates an electrical potential,
or voltage, at the battery terminals. This same principle
is used in the oxygen (O2) sensor to measure the oxygen
content in the exhaust gas, Figure 7-20. Many automotive
computer control systems rely heavily on the oxygen sen-
sor for much of the engine’s operating information.
Oxygen Sensor Construction
The sensor is made of a material, usually zirconia, that
reacts with oxygen to produce extra electrons. The zirconia
element is connected to internal electrodes, usually made
of platinum. The entire assembly is housed inside of a
steel shell. The difference in the amount of oxygen in the
exhaust gases and the outside air produces a weak elec-
trical current, which is sent to the ECM. Variations in the
oxygen content of the exhaust gases change the strength of
the electrical signal. The ECM reads changes in the signal
voltage as changes in the air-fuel ratio.
All OBD II–equipped vehicles use heated oxygen sen-
sors (HO2S). These sensors have an internal element that
preheats the sensor during vehicle warm-up. This allows
the computer control system to enter closed-loop operation
much more quickly.
Some oxygen sensors have titania cores. These sen-
sors modify voltage instead of producing it. This gives a
titania-core O2 sensor the ability to immediately send rel-
evant information to the ECM, rather than having to wait for
engine or sensor warm up.
Figure 7-18. Throttle position sensors are used to monitor the throttle position as it changes with driver input. (Subaru)
Not used
(full throttle
switch)
Idle
switch
Throttle
sensor
1
1
2
2
4
4
3
3
5
5
6
6
Supply voltage:
5V (applied between terminal No.4 and 6)
Throttle valve opening angle (deg)
0
0
4.0
6.0
2.0
Output voltage between
terminal No.4 and 5
Output
voltage
between
terminal
No.4
and
5(V)
45 90 135
Figure 7-19. This cutaway of a knock sensor shows the piezo-
electric element. This element vibrates and creates a small cur-
rent or modifi es reference voltage.
Piezoelectric
element
Diaphragm
Figure 7-20. The oxygen sensor is closely monitored by the
ECM to determine engine operating condition. If this sensor is
defective, it can affect the operation of the entire vehicle.
Oxygen Sensor
ECM