How Oxygen Sensors Work to Control Fuel Trim And Emissions

The O2 sensor

measures the oxygen content of the exhaust. The O2 sensor’s sensing ability
comes about by producing a small voltage proportionate to the exhaust oxygen
content. In other words, if the oxygen content is low it produces a high
voltage (0.90 Volts – Rich mixture) and if the oxygen content is high it
produces a low voltage (0.10 Volts – Lean mixture). Although theoretically the
O2 sensor should cycle between 0.00 volts and 1.00 volts, in reality it cycles
between 0.10 volts and 0.90 volts.
A GM 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
1) A few key issues are very important in the analysis of O2 sensor signals. 2) An O2 sensor
will cycle between 0.10 to 0.90 or almost 1 volt. 3) An O2 sensor has to reach
the 0.8x Volts amplitude mark while at full operation. 4) An O2 sensor also has
to reach the 0.1x Volts amplitude mark while at full operation. 5) 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. 6) The O2
sensor must cycle at least once per second, which would show 3 cross counts on
the scan tool PID. 7) Silicone is the leading cause of O2 contamination. 8) It
is easier for an O2 sensor to go from rich to lean than vise-versa. 9) O2
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 scale. 10) 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. 11)
Any time the O2 cycles and crosses the 0.450 volts mark, the system is in
CLOSE-LOOP. 12) Even though an O2 sensor is cycling and crossing 0.450 volts
(ECM in close loop) it DOES NOT mean that it is working properly. 13) O2 sensor
operation is extremely important not only to keep HC & CO emissions low but
also to the NOx as well. 14) 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
efficiency. 15) An O2 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.
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.
 
CONDITION 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
sensor signal 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 deceleration mode.
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 is 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.
COMPONENT
TESTING
NOTE:
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 sensors.
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.
           
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.
           
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.
           
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.
           
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.
  

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