basic o2 sensors
TRANSCRIPT
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Oxygen Sensors
Operation &
Testing
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Stoichiometry
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Stoichiometry
Stoichiometry in automotive terms is the principle that
refers to the ideal air to fuel ratio being consumed in
the cylinder.
At 14.7 pound of air to 1 pound of fuel, even pre-converter HC, CO and NOX exhaust gases will be at
their lowest levels.
However, no vehicle can maintain a precise 14.7:1mixture.
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Constant changes in the throttle position, engine load,
and vehicle speed cause the mixture to vary almost
constantly.
The feedback system has to adjust the mixtureconstantly to keep the mixture near optimum levels.
The system keeps the air/fuel mixture close to the
ideal by constantly switching back and forth on eitherside of stoichiometry.
Stoichiometry
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Oxygen SensorOperation
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Oxygen Sensor Operation
The oxygen sensor plays a critical role in providing
optimum engine performance with minimal emissions.
There are three basic types of oxygen sensors. Theyare the:
Zirconia
Titania Air Fuel
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Oxygen Sensor Operation
- Zirconia
The Zirconia oxygen sensor produces a voltage by
distributing oxygen ions across the surface of the two
platinum electrodes.
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Oxygen Sensor Operation
- Zirconia
An ion is an electrically charged atom. Oxygen ions
have a negative charge and are attracted to the ZrO2
electrolyte. Since the inside of the thimble-shaped
electrolyte is exposed to a much higher concentrationof oxygen ions than its outer side (exhaust side), a
potential difference is created between the two
platinum electrodes.
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The platinum is used as an electrical connection point.
It also serves as a catalyst for the chemical reaction
that enables the sensor to produce a voltage. It needs
about 599F (315
C) to completely set up the catalytic
reaction of the ZrO2 electrolyte.
At temperatures below 572F (300C), the difference
between rich and lean voltages diminishes rapidly.
This is the reason the computer ignores the oxygensensor signal until proper operating temperature is
achieved.
Oxygen Sensor Operation
- Zirconia
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A properly functioning zirconia oxygen sensor
generates a voltage up to one volt depending on the
oxygen content in the exhaust gas.
When the air/fuel ratio is lean, the oxygen content in
the exhaust gas is high. This will cause the oxygen
sensor to produce voltage less than 300 millivolts.
Conversely, there is less residual oxygen in the
exhaust gas when the mixture is rich. Under this
condition, the oxygen sensor will output a voltage in
excess of 600 millivolts.
Oxygen Sensor Operation
- Zirconia
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Oxygen Sensor Operation
- Zirconia
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Oxygen Sensor Operation
- TitaniaMany Titania sensors act similar to the Zirconia
sensor. The sensor uses a thick film of Titania at the
tip of the element to detect oxygen concentrations in
the exhaust gas.
They have less than one volt when rich and close to
five volts when lean.
Titania sensors are typically used on vehicles that arelikely to be driven off-road, because they do not
require a fresh air vent that can become clogged with
debris.
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Oxygen Sensor Operation
- Titania
Some Titania oxygen sensors, as used on the Toyota
Corolla GTS (except California) - and the V-6, 2WD
California truck, operate differently in that they have
close to one volt when lean and close to zero volts
when rich.
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Oxygen Sensor Operation
- Air Fuel (AF)The AF sensor, also called a broadband planar sensor
or Lean Air Fuel sensor (LAF), used in some
applications resembles the heated Zirconia sensor in
appearance only.
The AF sensor improves overall efficiency by keeping
the fuel control system in closed-loop during a much
wider range of driving conditions. Subsequently,
instead of using preprogrammed, open loop air/fuelratios in many situations, the PCM fine-tunes the
mixture much more closely based on the actual oxygen
readings from the exhaust.
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Oxygen Sensor Operation
- Air Fuel (AF)AF sensors may be configured as seven-wire, five wire,
or four-wire sensors.
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Oxygen Sensor Operation
- Air Fuel (AF)AF sensors are used as the pre-catalyst (upstream)
oxygen sensors.
On a four-wire AF sensor, two wires are power andground for the sensor heater, and the other two wires
are used for the exhaust mixture signal.
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Oxygen Sensor Operation
- Air Fuel (AF)The Air Fuel oxygen sensor heater is designed to heat
the oxygen sensor thimble to a minimum of 1200F.
This temperature is double that of an early four-wire
sensor and is required for the AF sensor to properlysample the exhaust oxygen content.
When the AF sensor heater is commanded on by the
PCM, approximately 8 amps of current should be
flowing through the circuit.
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Oxygen Sensor Operation
- Air Fuel (AF)The PCM controls the voltage to a fixed voltage. It is
difficult to confirm the AF sensor voltage without a scan
tool as the voltages at the terminals are fixed and any
change is noted within the PCM itself.
The voltage signal is proportional to the change in the
air/fuel mixture. This allows the PCM to more
accurately judge the exact air/fuel ratio under a wide
variety of conditions and quickly adjust the amount ofinjector pulse.
Think of the AF sensor as a generator that is capable of
changing polarity.
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Oxygen Sensor Operation
- Air Fuel (AF)The AF sensor has two zirconia elements that share a
diffusion chamber. The AF sensor is really two O2
sensors in one unit.
There are three chambers in the AF sensor:
The first chamber contacts the exhaust flow.
The diffusion chamber between the elements.
The air reference chamber.
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Oxygen Sensor Operation
- Air Fuel (AF)
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Oxygen Sensor Operation
- Air Fuel (AF)The first chamber is really the outside of the sensor,
which contacts the exhaust. The diffusion chamber is
the area between the two zirconia elements and the air
reference chamber is at the other end. The basicoperating principle behind the AF sensor is that by
controlling the amount of O2 in the diffusion chamber,
you can control its operating range.
One of the zirconia elements acts as an oxygen pump.We discussed how a flow of oxygen ions creates a flow
of electrons. The inverse is also true. A flow of
electrons applied to the sensor causes a flow of ions.
This is what happens when you charge a car battery.
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Oxygen Sensor Operation
- Air Fuel (AF)
Notice how the elements are wired in parallel, and there
is a common ground lead. This ground is a reference
point for the ECM.
Do not confuse it with the vehicle ground. In fact, if you
measure the voltage between the sensor ground and
the vehicle ground, you will see about 2.7 volts.
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Oxygen Sensor Operation
- Air Fuel (AF)
For this explanation, we will distinguish the two
zirconia elements by calling one, sensor #1 and the
other, sensor #2.
The ECM monitors the voltage between sensor #1
input and the ground lead. The ECM tries to hold the
voltage difference between sensor #1 and the ground
lead to 450 mV.
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Oxygen Sensor Operation
- Air Fuel (AF)
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Oxygen Sensor Operation
- Air Fuel (AF)
When the mixture goes rich, oxygen ions flow from the
diffusion chamber to the exhaust. The voltage on
sensor #1 input increases. The ECM detects the
voltage increase and reduces the voltage on sensor #2
input.
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Oxygen Sensor Operation
- Air Fuel (AF)
The voltage on sensor #2 input then goes more
negative than the ground voltage. This causes sensor
#2 to pump oxygen out of the diffusion chamber into
the air reference chamber. When the oxygen contentof the diffusion chamber drops, the voltage on sensor
#1 drops. At the same time that the ECM reduces the
voltage on sensor #2 input, it is also reducing fuel
delivery.
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Oxygen Sensor Operation
- Air Fuel (AF)When the mixture goes lean, oxygen ions flow from the
exhaust into the diffusion chamber. The voltage on
sensor #1 input decreases. The ECM detects the
voltage decrease and increases the voltage on sensor
#2 input. The voltage on sensor #2 input goes more
positive than the ground voltage. This causes sensor
#2 to pump oxygen into the diffusion chamber from the
air reference chamber. The voltage between sensor
#1 input and ground is consistently held at 450 mV.
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Oxygen Sensor Operation
- Air Fuel (AF)
The ECM knows how rich or lean the exhaust is by
how much amperage it takes the sensor #2 input to
hold sensor #1 input voltage to 450 mV.
Positive voltage the mixture is lean.
Negative voltage the mixture is rich.
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Oxygen Sensor Operation
- Air Fuel (AF)
Rich Mixture
Lean Mixture
Zero Volts
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Oxygen SensorHeater
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Oxygen Sensor Heater
To keep the engine operating in closed loop, an
oxygen sensor must be maintained at a minimum
temperature of approximately 572F (300C). To help
maintain this temperature, the oxygen sensors used on
OBD-II systems contain heating elements. These
elements combined with exhaust gas temperature
keep the oxygen sensor temperature at nearly 1200F.
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Oxygen Sensor Heater
The PCM monitors the operation of the internal oxygensensor-heating element. If the heater current exceeds
approximately 2 amps (standard system) or 8 amps
(AF system), the PCM will store a pending code and
enter freeze frame data.
Two consecutive failures of the Oxygen Sensor Heater
Monitor will result in:
A matured DTC MIL illumination
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Diagnosis of
Oxygen Sensor
Aging
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Oxygen Sensor Aging
To find out if the computer is "in control" of a vehicle's
air/fuel mixture, you need to view the oxygen sensor
signal and determine the computer command. Digital
Storage Oscilloscope (DSO) analysis will let youquickly determine whether the oxygen sensor is good
and whether the feedback computer system is in
control.
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Determining
System Control
& Condition
D t i i S t C t l
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Determining System Control
Signature Balance analysis is a term that describes the
appearance of the oxygen sensor waveform, and what
that waveform says about the vehicle's running
condition.
Most technicians already know the oxygen sensor
indicates engine mixture; what many technicians do
notknow is the oxygen sensor signal can show the
overall condition of the engine.
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D t i i S t C t l
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Determining System Control
Remember: It is this switching from rich to lean and
back again that sets up the conditions in the exhaustfor a three-way catalytic converter to oxidize HC and
CO and reduce NOX emissions efficiently.
D t i i S t C t l
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Determining System Control
Zero Volts
Determining S stem Control
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Determining System Control
If the engine is not running right, the sensor will not
develop a good waveform. If the oxygen sensor is
damaged, the engine will not run right.
So how can you tell whether the oxygen sensor
waveform is not right because of a bad sensor or an
engine problem?
Determining System Control
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Determining System Control
By verifying the oxygen sensor - forcing the system full
rich and full lean - you can check the maximum and
minimum voltage levels the sensor produces, and how
quickly it switches.
Slow switch rates from a deteriorated O2 sensor
causes poor performance and excessive exhaust
emissions. In some cases, the engine may run fine
but the vehicle will still fail an enhanced emissions test.
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Verifying
Zirconia
Sensors
Verifying Zirconia Sensors
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Verifying Zirconia Sensors
Connect a scope to the oxygen sensor signal wireand a good ground. Use the same ground as the
oxygen. This is critical to get an accurate
measurement.
Adjust your scope using these settings:
> Voltage Scale: 200 millivolts per division
> Time Base: 200 milliseconds per division
> Trigger Level: Auto On
> Lower the zero voltage level to one division
from the bottom of the display.
Verifying Zirconia Sensors
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Verifying Zirconia Sensors
Make sure the engine is at normal operating
temperature, and the oxygen sensor is fully
warmed up. (2500 RPM for about two minutes)
The engine must be in closed loop. If the
oxygen sensor voltage is not switching, it is not in
closed loop.
Verifying Zirconia Sensors
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Verifying Zirconia Sensors
Use your propane enrichment tool to richen the
engine, and see whether the oxygen sensor
voltage can rise above 800 millivolts.
Verifying Zirconia Sensors
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Verifying Zirconia Sensors
Quickly shut off the propane; this forces the
engine lean. Does the oxygen sensor voltage dropbelow 175 millivolts?
Verifying Zirconia Sensors
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Verifying Zirconia Sensors
While the engine is still lean, snap the throttle all
the way open, and check the oxygen sensorvoltage. The voltage should rise in less than 100
milliseconds.
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Verifying Zirconia Sensors
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Verifying Zirconia Sensors
Results:If the oxygen sensor voltage is fixed high, the
mixture is rich. Check for:
A dripping injector High fuel system pressure
A source of unmetered fuel such as the
canister purge system.
A problem in the computer system, such
as a miscalibrated coolant temperature
sensor.
Verifying Zirconia Sensors
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Verifying Zirconia Sensors
Results:A fixed low voltage signal indicates a lean mixture.
Check for:
Clogged injectors Low fuel pressure
A vacuum leak
A MAP or mass airflow sensor (MAF) that
is out of calibration.
Verifying Zirconia Sensors
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Verifying Zirconia Sensors
Misfire:One of the most obvious failures to show up on the
oxygen sensor signal is a misfire in the engine.However, few technicians realize just how clearly a
misfire will appear on the oxygen sensor signal.
Verifying Zirconia Sensors
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Verifying Zirconia SensorsMisfire:
The graphic shows what a misfire will look like onthe oxygen sensor signal - a high frequency
variation, bouncing high and low, much faster than
a normal oxygen sensor signal.
Verifying Zirconia Sensors
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Verifying Zirconia Sensors
Misfire:The misfire forces a pulse of air past the oxygen
sensor, which disrupts the exhaust surrounding the
sensor. The rapid change from high oxygen to lowoxygen, and back again causes the sensor to read
a rapid change in the exhaust oxygen sensor, and
the sensor develops a high frequency signal, such
as the one shown.
Verifying Zirconia Sensors
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Verifying Zirconia Sensors
Misfire:Of course, this depends on the rest of the system
being in proper control of the mixture. A misfire
may not show up at all on a system with the oxygensensor signal fixed rich or lean. However, if the
sensor is switching properly, a misfire will show up
on the oxygen sensor signal.
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Honda's Air Fuel Sensor
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Honda s Air Fuel Sensor
When a vehicle travels at a steady cruising speed,
it can achieve much better fuel efficiency and bettergas mileage with a mixture that is leaner than
14.7:1. Engineers designed an oxygen sensor to
take advantage of this better fuel efficiency. One
Honda model that uses the Air Fuel (AF) sensor isa lean-burn Civic.
The AF sensor's design lets it hold the mixture at
14.7:1 during some driving conditions and leaner
than 14.7:1 during other conditions. This AF sensor
can monitor exhaust O2 levels over a greater range.
Its total operating range is from about 14:1 up to
23:1.
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Air Fuel
Sensor Testing
- Honda
Air Fuel Sensor Testing - Honda
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Air Fuel Sensor Testing - Honda
Using a DSO Check for any trouble codes. Repair the
problem and check to see if a road test resets the
code.
Connect the positive lead of your scope to
second sensor input (the white sensor wire) and the
meter ground lead to the sensor ground lead (the
black sensor wire, not the vehicle ground).
Air Fuel Sensor Testing - Honda
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Using a DSO
Adjust your scope using these settings:
> Voltage Scale: 500 millivolts per division
> Time Base: 200 milliseconds per division
> Trigger Level: Auto On
> Put the zero voltage level in the middle of the
display.
Air Fuel Sensor Testing - Honda
Air Fuel Sensor Testing - Honda
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Using a DSO
Make sure the engine is at normal operatingtemperature, and the AF sensor is fully warmed up.
(2000 RPM for about two minutes)
Air Fuel Sensor Testing - Honda
Air Fuel Sensor Testing - Honda
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Using a DSO
Use your propane enrichment tool to richen the
engine, and see whether the AF sensor voltage
goes negative. Record the average low voltage.
Air Fuel Sensor Testing - Honda
Zero Volts
Rich Mixture
Air Fuel Sensor Testing - Honda
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Using a DSO
Quickly shut off the propane, and force the
engine lean. See whether the AF sensor voltage
goes positive. Record the average high voltage.
Air Fuel Sensor Testing - Honda
Zero Volts
Lean Mixture
Air Fuel Sensor Testing - Honda
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Using a DSO
Drop the negative sign on the negative voltage,
then add the two voltage values together to get thedifference between high and low voltages. The
total difference should be over 1 volt. If it is not
over one volt, the AF sensor may be bad.
Air Fuel Sensor Testing - Honda
Air Fuel Sensor Testing - Honda
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Using a DSO
While the engine is
still lean, snap the
throttle all the way
open, and watch the
AF sensor voltage.
The voltage should
fall in less than 100
milliseconds. If it
takes longer than 100ms, the AF sensor
may be contaminated
or bad.
Air Fuel Sensor Testing - Honda
Zero
Volts
Air Fuel Sensor Testing - Honda
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Using a DSOIf the AF sensor signal meets these requirements,
the sensor is probably okay. If not, replace the
sensor, and verify its operation again.
Once you verify the AF sensor signal, look for any
problems the signal indicates.
Air Fuel Sensor Testing Honda
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Air Fuel
Sensor Testing
- Toyota
Air Fuel Sensor Testing - Toyota
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Using a Scan Tool
Add external fuel at idle and check O2 voltage.When external fuel is added to the intake manifold,
the scan tool displays oxygen sensor voltage
dropping for a split second to 0.640 volts. Then the
voltage returns to 0.650 volts even with fuel stillbeing added.
Create a lean condition and check O2 voltage.
When the power brake booster vacuum hose is
removed, the scan tools oxygen sensor voltage
increases for a split second to 0.670 volts then the
voltage returns to 0.650 volts even with the booster
vacuum hose still removed.
Air Fuel Sensor Testing Toyota
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