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Oxygen Sensors Operation & Testing


Stoichiometry


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 preconverter HC, CO and NOX exhaust gases will be at their lowest levels. However, no vehicle can maintain a precise 14.7:1 mixture.


Stoichiometry 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 mixture constantly 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 either side of stoichiometry.


Stoichiometry The engine management system is responsible for maintaining stoichiometry allowing the catalytic converter to operate at maximum efficiency while maintaining minimal emissions. The engine management system uses an oxygen sensor in the exhaust to determine if the air/fuel mixture is rich or lean and adjusts to compensate.


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Oxygen Sensor Operation


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. They are the: • • •

Zirconia Titania Air Fuel


Oxygen Sensor Operation - Zirconia The Zirconia oxygen sensor produces a voltage by distributing oxygen ions across the surface of the two platinum electrodes.


Oxygen Sensor Operation - Zirconia An ion is an electrically charged atom. Oxygen ions have a negative charge and are attracted to the ZrO 2 electrolyte. Since the inside of the thimble-shaped electrolyte is exposed to a much higher concentration of oxygen ions than its outer side (exhaust side), a potential difference is created between the two platinum electrodes.


Oxygen Sensor Operation - Zirconia 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 599째F (315째C) to completely set up the catalytic reaction of the ZrO2 electrolyte. At temperatures below 572째F (300째C), the difference between rich and lean voltages diminishes rapidly. This is the reason the computer ignores the oxygen sensor signal until proper operating temperature is achieved.


Oxygen Sensor Operation - Zirconia 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 - Titania Many 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 are likely to be driven off-road, because they do not require a fresh air vent that can become clogged with debris.


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 rich and close to zero volts when lean.


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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/fuel ratios in many situations, the PCM fine-tunes the mixture much more closely based on the actual oxygen readings from the exhaust.


Oxygen Sensor Operation - Air Fuel (AF) AF sensors may be configured as seven-wire, five wire, or four-wire sensors.


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 and ground for the sensor heater, and the other two wires are used for the exhaust mixture signal.


Oxygen Sensor Operation - Air Fuel (AF) The Air Fuel oxygen sensor heater is designed to heat the oxygen sensor thimble to a minimum of 1200째F. This temperature is double that of an early four-wire sensor and is required for the AF sensor to properly sample 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.


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 of injector pulse. Think of the AF sensor as a generator that is capable of changing polarity.


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.


Oxygen Sensor Operation - Air Fuel (AF)


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 basic operating 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.


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.


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.


Oxygen Sensor Operation - Air Fuel (AF)


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.


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 content of 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.


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.


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.


Oxygen Sensor Operation - Air Fuel (AF)

Lean Mixture Zero Volts

Rich Mixture


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Oxygen Sensor Heater


Oxygen Sensor Heater To keep the engine operating in closed loop, an oxygen sensor must be maintained at a minimum temperature of approximately 572째F (300째C). 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 1200째F.


Oxygen Sensor Heater The PCM monitors the operation of the internal oxygen sensor-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


Oxygen Sensor Aging Age, contamination, and extreme heat can affect the oxygen sensor response characteristics. Degradation of the signal can be in the form of an extended response time (period duration) or a shift in the sensor voltage curve (sensor shift biased). Both conditions reduce the oxygen window, thereby reducing the catalyst capacity for exhaust gas conversion.


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 you quickly determine whether the oxygen sensor is good and whether the feedback computer system is in control.


Determining System Control & Condition


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 not know is the oxygen sensor signal can show the overall condition of the engine.


Determining System Control

In general, the oxygen sensor waveform should appear like the waveform in the graphic to be shown. They must switch continuously above and below 450 millivolts, switching between once every two seconds, and five times per second. In addition, it should not drop below zero volts, except from occasional noise. However, the AF sensor is an exception to this rule.


Determining System Control

Remember: It is this switching from rich to lean and back again that sets up the conditions in the exhaust for a three-way catalytic converter to oxidize HC and CO and reduce NOX emissions efficiently.


Determining System Control

Zero Volts


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 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.


Verifying Zirconia Sensors


Verifying Zirconia Sensors • Connect a scope to the oxygen sensor signal wire and 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


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 • Use your propane enrichment tool to richen the engine, and see whether the oxygen sensor voltage can rise above 800 millivolts.


Verifying Zirconia Sensors • Quickly shut off the propane; this forces the engine lean. Does the oxygen sensor voltage drop below 175 millivolts?


Verifying Zirconia Sensors • While the engine is still lean, snap the throttle all the way open, and check the oxygen sensor voltage. The voltage should rise in less than 100 milliseconds.


Verifying Zirconia Sensors

Results: If the oxygen sensor signal meets these requirements, the sensor is probably okay. If not, replace the sensor, and verify its operation again.


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 •


Verifying Zirconia Sensors Results: A fixed low voltage signal indicates a lean mixture. Check for: • Clogged injectors • Low fuel pressure •


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 Misfire: The graphic shows what a misfire will look like on the oxygen sensor signal - a high frequency variation, bouncing high and low, much faster than a normal oxygen sensor signal.


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 low oxygen, 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 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 oxygen sensor signal fixed rich or lean. However, if the sensor is switching properly, a misfire will show up on the oxygen sensor signal.


Honda's Air Fuel Sensor


Honda's Air Fuel Sensor When a vehicle travels at a steady cruising speed, it can achieve much better fuel efficiency and better gas 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 is a 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.


Air Fuel Sensor Testing - Honda


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 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

Using a DSO • Make sure the engine is at normal operating temperature, and the AF sensor is fully warmed up. (2000 RPM for about two minutes)


Air Fuel Sensor Testing - Honda 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


Air Fuel Sensor Testing - Honda 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.

Lean Mixture Zero Volts


Air Fuel Sensor Testing - Honda

Using a DSO • Drop the negative sign on the negative voltage, then add the two voltage values together to get the difference 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 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 100 ms, the AF sensor may be contaminated or bad.

Zero Volts


Air Fuel Sensor Testing - Honda

Using a DSO If 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 - Toyota


Air Fuel Sensor Testing - Toyota 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 still being added. • Create a lean condition and check O2 voltage. When the power brake booster vacuum hose is removed, the scan tool’s 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 Using a Scan Tool The average generic OBD-II scan tool voltage indicating stoichiometric fuel trim for these vehicles is 0.660 volts. The minimum rich mixture voltage indication from the oxygen sensor reading on the scan tool when adding external fuel should be 0.56 volts. The minimum lean mixture voltage indication from the oxygen sensor reading on the scan tool when inducing a large vacuum leak should be 0.76 volts.


Air fuel Ratio Sensor and O2 Sensors Operation  

This is a quck description on the sensor operation in the smog class.

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