A few key issues are very important in the analysis
of O2 sensor signals.
• An O2
sensor will cycle between 0.10 to 0.90 or almost 1 volt.
• An O2
sensor has to reach the 0.8x Volts amplitude mark while at full
sensor also has to reach the 0.1x Volts amplitude mark while at full
Full operation means the engine is fully warmed up,
O2 sensor above the 600 deg. F. operating temperature, and no fuel
or mechanical problems present.
O2 sensor must cycle at least once per second, which
would show 3 cross
counts on the scan tool PID.
Silicone is the leading cause of O2 contamination.
easier for an O2 sensor to go from rich to lean than vise-versa.
sensors tend to fail on rich bias. In other words, they tend to
shift their cycling to the upper side or rich side of the voltage
Contrary to what many people think, an O2 sensor WILL NOT cycle by
itself. The O2 sensor cycle is a direct result of the ECM response
to the changes in the mixture.
time the O2 cycles and crosses the 0.450 volts mark, the system is
though an O2 sensor is cycling and crossing 0.450 volts (ECM in
close loop) it DOES
NOT mean that it is working properly.
sensor operation is extremely important not only to keep HC & CO
emissions low but also to the NOx as well.
Proper O2 sensor cycling will determine the catalytic converter’s
efficiency. The catalytic converter needs the O2 sensor cycling at
its proper amplitude and frequency for it to function at its maximum
sensor with a high voltage reading does not necessarily mean that
the mixture is rich or high in fuel content. An EGR valve problem
will send the O2 signal high as well.
O2 sensor signal stuck at 450 mV is an indication of an open O2 sensor circuit
(signal wire) or
faulty O2 signal ground. The 450 mV value (GM) is called a bias
voltage and it is not the same for all manufacturers. Some
manufacturers employ a dedicated O2 sensor ground. Such a ground
lead is attached to the engine block or chassis and feeds an ECM O2
ground pin only. The O2 circuit is then grounded through the inside
of the ECM electronic board by this ground wire. A loss of this
ground would also put the O2 sensor signal at around 450 mV, which
also makes it look like an open circuit. The same holds true for
Chrysler, but these use a different O2 bias voltage, which is
usually 2.00 to 4.00 volts.
A big misconception among technicians trying to understand
O2 sensors is that they cycle by themselves. The O2 sensor just reads oxygen
content in the exhaust,
THAT’S IT. Excess oxygen in the form of regular ambient air will send
the O2 sensor voltage signal low (under 0.450 volts) and lack of it will
send the voltage signal high (over 0.450 volts). A stuck open EGR valve will
create a lack of oxygen in the exhaust, since the re-circulating exhaust has
all its oxygen already burnt . The ECM sometimes uses the O2 sensor to check
for proper EGR operation and sets a code if necessary. So, be aware of the
fact that a vehicle might be running lean because the ECM sees a rich O2
signal due to a defective (stuck open) EGR valve. Since the ECM sees a rich
signal, it will try to correct with a lean command and try to lower the O2
sensor’s high voltage signal.
Conditions that Affect Operation
NOTE: WHEN PERFORMING O2 SENSOR CHECKS, IT
IS IMPORTANT TO TAKE MEASUREMENTS AT IDLE AND 2000 RPM. BE AWARE THAT O2
SENSOR PRE-CONDITIONING IS IMPORTANT, EVEN ON THE NEWER STYLED HEATED O2
SENSORS. PRE-CONDITION THE O2 SENSOR BY RAISING THE ENGINE SPEED TO 2000 RPM
FOR ABOUT 15 SECONDS OR SO. THE O2 SENSOR HAS TO BE ABOVE 600 º F. TO BE
ABLE TO OPERATE PROPERLY. LONG PERIODS OF IDLE TIME CAN RENDER A NON-HEATED
OR OLDER O2 SENSOR TOO COLD FOR IT TO FUNCTION AT ALL. AT THE SAME TIME, DO
NOT TRY TO FORCE A HEATED O2 SENSOR INTO OPERATION. AN O2 SENSOR WITH A
FAULTY HEATER WILL GO INTO CLOSED-LOOP AFTER A GOOD WARM-UP SESSION.
After an engine has ran through its warm up period (O2
sensor has no effect on engine operation while the engine is cold), the ECM
then looks for the O2 value. The 0.450 volts mark is considered almost
universally as the midway point or crossover point for O2 sensor operation.
If the signal is on the rich side (above 0.45 volts), then the ECM will
answer with a lean command (reducing injector pulsation), or if the signal
is on the lean side (below 0.45 volts) then the ECM
will answer with a rich command (increasing injector
pulsation). The amount of injector pulse correction is
proportional to the voltage
seen by the ECM at the O2 sensor signal wire. The higher the voltage the
more the ECM reduces on-time to the injector. The lower the voltage the more
the ECM increases the injector on-time. The ECM is constantly doing exactly
just that, slightly increasing and decreasing injector pulsation. The
constant adjustment is what gives the O2 sensorsignal the switching
appearance (sine wave) on the scope screen.
NOTE: The ECM’s fuel pulse
corrections performed constantly to the injector signal is called SHORT TERM
FUEL TRIM ( GM called it INTEGRATOR ) and LONG TERM FUEL TRIM ( GM called it
BLOCK LEARN ) on the scanner. FUEL TRIMS is the system’s deviation of the
BASE-INJECTION pulse. Analyzing LTFT and STFT is a great way to know a
particular vehicle’s fuel consumption trend or how well that vehicle has
been performing with regards to fuel control. STFT and LTFT is the first
thing to look for when assessing fuel control problems.
The fact that the O2 sensor signal is switching
rich-lean-rich-lean also reveals that the ECM is controlling the injector
pulsation and therefore that the system is in
close loop mode. An ECM in full
control (O2 sensor cycling) is said to be in close loop because of the close-circuit action of
O2 sensor-to ECM-to injector pulse control then to O2 sensor and back to the
ECM. The ECM must be in control at all times except during
warm up, WOT, power enrichment, and
The O2 sensor not only has to cycle, it also has to cycle
fast enough (proper frequency) and wide enough (proper amplitude). At least
one cycle per second ( 1 Hz
) must be seen at the signal wire in order for the O2 to be considered
good (not lazy). A one cycle per second will make the scope trace go across
the 0.450 volts mark approximately 3 times, which the ECM recognizes as 3
cross counts. A slow O2 sensor will have a damaging effect on the catalytic
converter and release excessive amounts of emissions to the atmosphere.
A cycle are the complete rich and lean crests of the O2
sensor signal, while crossing the 0.45 voltage point. Proper amplitude
refers to the O2 sensor’s ability to reach full rich ( 0.90 volts ) and full
lean ( 0.10 volts ) when cycling. The higher the voltage seen at the O2
signal line the more the ECM reduces pulsation to the injectors. The lower
the voltage seen at the O2 signal line the more the ECM increases injector
pulsation. This is the reason why an O2 sensor that is not reading the
mixture properly, at full amplitude and frequency, will actually misguide
the ECM into a wrong fuel control pattern. Once the O2 sensor has reached
its correct temperature of 600 º F, look for an O2 signal cycle with the
correct amplitude and frequency and it will surely indicate a perfectly
operating O2 sensor.
On early OBD II systems, the post
catalytic converter O2 sensor has no effect on fuel control. The post
catalytic O2 sensor was originally responsible for only monitoring catalytic
converter efficiency. On most systems, the post converter O2 sensor signal
should never mimic or follow the pre-cat O2 signal. That would indicate a
defective or low oxygen storage capability at the converter . On early OBD
II systems, the post-cat O2 sensor should show little or no voltage
fluctuations on a scope waveform, since all the mixture fluctuations are
being absorbed by the catalytic converter.
Stating around model year 1999, a new type of converter came
on the market, called “Low Oxygen Storage Converter” or LOC. With an LOC,
the pre and post O2 sensors cycle at the same rate. These converters are
tested by measuring the lag-time between the two signals. A further
development of this system is that the post converter signal is also used
for A/F correction, but to a less extent.
These simple steps should be followed whenever testing O2
1. Scan the vehicle for any O2 sensor codes and analyze the
data stream PID. O2 sensor voltage should cycle normally with proper
amplitude and frequency. An O2 sensor stuck at a fixed bias voltage is an
indication of an open O2 circuit or lack of O2 sensor (dedicated) ground. If
possible use a graphing multi-meter to analyze the O2 sensor data to
determine any possible problems.
2. While reading the scan values, goose the throttle and
observe for O2 sensor minimum and maximum values (0.1x volts to 0.9x volts).
Although this is not a conclusive evidence of correct O2 sensor operation,
it serves as a preliminary indication of proper operation.
3. Some automotive manufacturers employ a dedicated O2
sensor ground wire that is grounded somewhere at the engine block or
chassis. A loss or rupture of this ground wire will render the O2 sensor
useless. This ground wire feeds only the ECM’s O2 sensor circuit. The main
engine ground does not feed this type of O2 sensor circuit.
4. Verify the O2 sensor wire integrity. Most O2 sensors are
biased and an open signal wire will give a reading of whatever the bias
voltage is. Later model Jeep/Chrysler O2 circuits tend to be biased at
around 2 or 4 volts, therefore, a constant reading of around 2 or 4 volts on
a Chrysler is also an indication of an open circuit. In many of these cases,
the ECM will put an “O2 sensor High Voltage” code.
5. Finally, verify for correct O2 sensor operation with a
scope or graphing multi-meter. Check for proper amplitude and frequency.
Remember that the scanner O2 sensor readings are only interpreted values and
may not show the real voltage reading. This is the reason for doing this
final manual test.