E Meyer wrote:
> On 3/16/06 6:16 AM, in article
> ,
> " > wrote:
>
> > Re. O2 sensors: if you decide to replace an O2 sensor make sure you
> > replace all the O2 sensors on your car at the same time.
>
> Why?
>
Unless you know the other O2s are operating as well as the new sensor,
how can you predict how the on board computer will interpret the
different inputs from the new vs old O2 sensors? (Use of a scan tool
can eliminate this uncertainly, of course.) Is there a doubt about
sensor performance degradation over time?

excerpted from
http://www.autotap.com/articles/Unde...n_Sensors.html

You can read the O2 sensor's output with a scan tool or digital
voltmeter, but the transitions are hard to see because the numbers jump
around so much. Here's where a PC based scantool such as AutoTap really
shines. You can use the graphing features to watch the transitions of
the O2 sensors voltage. The software will display the sensor's voltage
output as a wavy line that shows both it's amplitude (minimum and
maximum voltage) as well as its frequency (transition rate from rich to
lean).

A good O2 sensor should produce an oscillating waveform at idle that
makes voltage transitions from near minimum (0.1 v) to near maximum
(0.9v). Making the fuel mixture artificially rich by feeding propane
into the intake manifold should cause the sensor to respond almost
immediately (within 100 milliseconds) and go to maximum (0.9v) output.
Creating a lean mixture by opening a vacuum line should cause the
sensor's output to drop to its minimum (0.1v) value. If the sensor
doesn't flip-flop back and forth quickly enough, it may indicate a need
for replacement.

If the O2 sensor circuit opens, shorts or goes out of range, it may set
a fault code and illuminate the Check Engine or Malfunction Indicator
Lamp. If additional diagnosis reveals the sensor is defective,
replacement is required. But many O2 sensors that are badly degraded
continue to work well enough not to set a fault code-but not well
enough to prevent an increase in emissions and fuel consumption. The
absence of a fault code or warning lamp, therefore, does not mean the
O2 sensor is functioning properly.

end excerpt