Basic O2 Sensors

7/31/2019 Basic O2 Sensors

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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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- Zirconia

The Zirconia oxygen sensor produces a voltage by

distributing oxygen ions across the surface of the two

platinum electrodes.


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- 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.


- 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.


- Zirconia


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- Zirconia


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- 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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- 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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- 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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- Air Fuel (AF)AF sensors may be configured as seven-wire, five wire,

or four-wire sensors.


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- 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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- 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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- 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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- 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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- Air Fuel (AF)


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- 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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- 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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- 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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- Air Fuel (AF)


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- 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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- 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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- 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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- 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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- 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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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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Zero Volts

Determining S stem Control


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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?



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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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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.



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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.



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Use your propane enrichment tool to richen the

engine, and see whether the oxygen sensor

voltage can rise above 800 millivolts.



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Quickly shut off the propane; this forces the

engine lean. Does the oxygen sensor voltage dropbelow 175 millivolts?



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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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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.



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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.



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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.



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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.



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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.



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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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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).



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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.




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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)




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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.


Zero Volts

Rich Mixture



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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.


Zero Volts

Lean Mixture



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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.




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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.


Zero

Volts



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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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Date post:	05-Apr-2018
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Basic O2 Sensors

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