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An Oxygen sensor is a chemical generator. It is
constantly making a comparison between the Oxygen inside the exhaust
manifold and air outside the engine. If this comparison shows little
or no Oxygen in the exhaust manifold, a voltage is generated. The
output of the sensor is usually between 0 and 1.1 volts. All spark
combustion engines need the proper air fuel ratio to operate
correctly. For gasoline this is 14.7 parts of air to one part of
fuel. When the engine has more fuel than needed, all available
Oxygen is consumed in the cylinder and gasses leaving through the
exhaust contain almost no Oxygen. This sends out a voltage greater
than 0.45 volts. If the engine is running lean, all fuel is burned,
and the extra Oxygen leaves the cylinder and flows into the exhaust.
In this case, the sensor voltage goes lower than 0.45 volts. Usually
the output range seen seen is 0.2 to 0.7 volts.
The sensor does not begin to generate it's full output until it
reaches about 600 degrees F. Prior to this time the sensor is not
conductive. It is as if the circuit between the sensor and computer
is not complete. The mid point is about 0.45 volts. This is neither
rich nor lean. A fully warm O2 sensor *will not spend any time at
0.45 volts*. In many cars, the computer sends out a bias voltage of
0.45 through the O2 sensor wire. If the sensor is not warm, or if
the circuit is not complete, the computer
picks up a steady 0.45 volts. Since the computer knows this is an
"illegal" value, it judges the sensor to not be ready. It remains in
open loop operation, and uses all sensors except the O2 to determine
fuel delivery. Any time an engine is operated in open loop, it runs
somewhat rich and makes more exhaust emissions. This translates into
lost power, poor fuel economy and air pollution.
The O2 sensor is constantly in a state of transition between high
and low voltage. Manufacturers call this crossing of the 0.45 volt
mark O2 cross counts. The higher the number of O2 cross counts, the
better the sensor and other parts of the computer control system are
working. It is important to remember that the O2 sensor is comparing
the amount of Oxygen inside and outside the engine. If the outside
of the sensor should become blocked, or coated with oil, sound
insulation, undercoating or antifreeze, (among other things), this
comparison is not possible.
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