Driveability Corner

Driveability Corner
Will a scan tool’s sample rate and the PCM’s reporting capabilities
allow you to reliably test a conventional zirconia dioxide oxygen
sensor? The answer depends on when and how you run your tests.
Mark
Warren
smwarren@motor.com
I
s a generic scan tool sample rate
fast enough to test a conventional
zirconia dioxide oxygen sensor?
Let’s take a look to find out. Fig. 1
is a recording of bank 1 sensor 1
on a 2000 Honda Odyssey with a
3.5L engine. The recording was set up to
capture just the bank 1 sensor and no other
parameters.
During this recording I was holding the
throttle at about 2000 rpm (80 to 100 seconds on the scale at the top of graph), then
snapped the throttle wide open and shut
(100 to 110 seconds). I repeated the same
procedure between 115 and 140 seconds.
When snap-testing an O 2 sensor, what
data is important?
•Minimum and maximum voltage.
•Rich-to-lean and lean-to-rich transition
times.
•Does the O 2 sensor exhibit negative
voltage? If it does, how much?
•What is the switch frequency at 2000
rpm?
•Is the scan tool’s sample rate sufficient?
Fig. 2 is a zoom in on Fig. 1. You can see
that the green cursor is in the same location in both figures, at the 100.8-second
point. Fig. 2 is configured to show the actual data points, with graph lines drawn in
between the data points.
Looking at Fig. 2, let’s answer the O 2
sensor test questions. Minimum voltage is
zero. When testing an O 2 sensor with a
continued on page 18
Fig. 2
Graphs: Mark Warren
Fig. 1
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June 2012
Driveability Corner
Fig. 3
snap test, voltage must drop below
.200 or the sensor must be replaced. Maximum voltage is .940
volt; the sensor must go above .800
volt or be replaced.
The rich-to-lean and lean-to-rich
transition times are measured from
.800 to .200 and from .200 to .800
volt. The transition time on a good
sensor is less than 150 milliseconds
(mS), or .150 second. Note that the
time in seconds at the top of the
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18
June 2012
graph is .4 second per major division. The switch times from rich to
lean and lean to rich are reasonably close to 100 to 150mS.
Does this O2 sensor exhibit negative voltage? Okay, most vehicle
PCMs do not report negative voltages to the scan tool, but some do.
The fact that the O 2 sensor pegs on
zero for about 2 seconds on the
lean transition leads me to believe
that some negative voltage may be
present. How much is too much,
and how do I know? The best testing method is with a scope, because sample rates are way better
and negative voltage is easy to
quantify.
But we’re trying to get the
fastest, easiest test using a scan
tool here. Sensor makers and vehicle manufacturers usually specify
that no negative voltage is acceptable. However, I’ve seen negative
voltages on brand-new vehicles.
Using Fig. 2 and counting the
ple rate is fast enough for this type
Experience shows that up to
sample points that occur in 10 secof O2 sensor analysis.
150mV negative is okay but below
Fig. 3 on the previous page
onds, we can see that we’re getting
that, replacement is in order. As
shows a graph of a similar capture
about seven samples per second, or
oxygen sensors shift negative, the
on the same vehicle while recordabout 150mS between samples.
maximum voltage
ing 10 data parameters.
generally drops proI put up rpm as well
p o r t i o n a l l y. T h e r e so you could see the
fore, when very negaThe best testing method is with snap-throttle events.
tive at the minimum
Other than min/max
a scope, because sample rates
voltage, the sensor
voltages, no other oxywill usually not
gen sensor analysis is
are much better and negative
achieve the required
possible because the
max voltage.
additional parameters
voltage is easy to quantify.
The switch frehave slowed the samquency is about once
ple rate. Also, note the
angular nature of the
per second, or 1Hz.
graph; when the sample rate is too
We have a sufficient data sample
This is just enough to get a pretty
rate to analyze the switch rate.
slow, this is a good tip-off that samgood idea about transition times.
Switch rate is a function of O2 senpling is very slow.
Remember, this is the ISO commuKnowing your sample rate and
sor response combined with the
nication OBD II protocol. Some
the speed of your signal will make
fuel system’s response rate. Note
may be slower and some may be
that the rate is slower at idle speed
diagnostics easier and help you
faster, and you’ll have to look at
avoid a misdiagnosis.
and is rpm-dependent.
the data to determine if your sam-
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Circle #16
June 2012
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