Chapter
C
: Fuel, Air Induction, and Exhaust Systems Diagnosis and Repair
64
Oxygen Sensor Testing
To determine the voltages the oxygen sensor produces,
use a DMM capable of displaying minimum, maximum,
and average voltage readings, or an oscilloscope. To
test with a DMM, connect the positive meter lead to the
HO2S signal wire and connect the negative lead to a re-
liable ground. Be careful not to pull at the sensor wire or
to allow it to make contact with a hot exhaust manifold.
Also, beware of puncturing a grouping of signal wires
with a common grounding shield. Doing so may ground
out the sensor.
Next, run the engine until the HO2S reaches operating
temperature and the control system switches from
open-loop to closed-loop operation. Once the system is
in closed-loop, the DMM should sample a range of oxy-
gen sensor voltages similar to that produced during a
real-life duty cycle. Select the “minimum-maximum”
feature on the DMM, then raise engine speed to 2,000
rpm for 30 seconds. Return engine speed to idle, then
quickly snap open and release the throttle once. Switch
off the minimum-maximum feature.
For a properly functioning engine, the DMM should dis-
play a minimum voltage below 200 mV, a maximum volt-
age of at least 700 mV, and an average of approximate-
ly 450 mV. If voltages differ, refer to an HO2S diagnostic
chart, figure 3-19. Please note that this procedure tests
a system, not an individual component. Incorrect volt-
ages do not always indicate a faulty oxygen sensor.
For more accurate HO2S test results, monitor sensor
voltage with an oscilloscope. The PCM looks for the
oxygen sensor voltage signal to change direction near
the 450 mV midpoint of the range. Ideally, the signal is
evenly split and stays above and below 450 mV equal
amounts of time. This indicates an oxygen content that
is equivalent to the ideal stoichiometric air-fuel ratio of
14.7:1, figure 3-20. When exhaust oxygen content is
high, a zirconia HO2S generates a low-voltage signal. A
low exhaust oxygen content causes the HO2S to pro-
duce a high-voltage signal. An HO2S signal that remains
below longer than it stays above 450 mV indicates a
lean condition, and a waveform that spends more time
above 450 mV indicates a rich running condition. The
farther from the voltage midpoint the switch occurs, the
more severe the mixture deviation.
VOLTAGE READINGS
COURSE OF ACTION
Minimum
Maximum
Average
Over 700 mV
400-500 mV
Over 200 mV
400-500 mV
400-500
mV
Over 700 mV
Over 700 mV
Over 500 mV
Over 200 mV
Over 700 mV
None - System operating
properly.
Replace oxygen sensor.
Replace oxygen sensor.
Check for cause of lean
air-fuel ratio.
Not applicable
Check the oxygen sensor by
enriching the air-fuel mixture.
If the sensor responds,
investigate for a lean running
engine. If the sensor does not
respond, replace the sensor.
Investigate cause or rich
running engine.
Check the sensor by leaning
out the air-fuel mixture. If the
Sensor responds, investigate
for a rich running engine. If it
does not respond, replace it.
Under 400 mV
Under 700 mV
Under 200 mV
Under 700 mV
Not applicable
Under 200 mV
Over 500 mV
Under 200 mV
Under 200 mV
Under 400 mV
Fig. 3-19.
Oxygen sensor diagnostic chart.
Summary of Contents for ASE-A8
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