AFR sensor
Encyclopedia
The AFR sensor is an air-fuel ratio
sensor that is slowly replacing the Zirconium
oxygen sensor
(O2 sensor) in modern motor vehicles.
, which makes it a type of generator. The generated varying voltage shows up on the scope
as the familiar 1 Hz
sine wave
, when in closed loop control. The actual voltage that is generated is the difference between the O2 content of the exhaust and that of the surrounding ambient air. The stoichiometric air-fuel ratio
or the mixture of air-to-fuel equal to 14.7:1 is theoretically the best mixture ratio for gasoline engines.
This is the theoretical air -to- fuel ratio at which all of the fuel will react with all of the available oxygen resulting in complete combustion. At or near this ratio, the combustion
process produces the best balance between power and low emissions. At a stoichiometric air-fuel ratio (14.7:1), the generated O2 sensor voltage is about 450 mV. The Engine Control Module (ECM) recognizes a rich condition above the 450 mV level, and a lean condition below it, but does not detect the extent of the richness or leanness. It is for this reason that the Zirconium O2 sensor is called a “narrow-band” O2 sensor.
O2 sensor was used throughout the late 1980s and early 1990s on a limited basis. This sensor’s semiconductor
construction makes its operation different from that of the Zirconium O2 sensor. Instead of generating its own voltage, the Titanium O2 sensor’s electrical resistance
changes according to the exhaust oxygen content. When the air/fuel ratio is rich, the resistance of the sensor is around 950 Ohm
s and more than 21 Kilohms when the mixture is lean. As with the Zirconium sensor, the Titanium O2 sensor is also considered a narrow-band O2 sensor.
sensors (lambda representing air-fuel ratio
), AFR (air-fuel ratio sensors), LAF (lean air-fuel sensor) and wide-band O2 sensor. Regardless of the name, the principle is the same, which is to put the ECM in a better position to control the air/fuel mixture. In effect, the wide-band O2 sensor can detect the exhaust’s O2 content way below or above the perfect 14.7:1 air/fuel ratio. Such control is needed on new lean burn
ing engines with extremely low emission output levels. The tighter emission regulations are actually driving this newer fuel control technology and in the process making the systems much more complex and difficult to diagnose.
If on the other hand the air/fuel mixture goes rich, the pump cell circuit voltage rapidly climbs high and the ECM immediately reverses the current polarity to readjust the pump cell circuit voltage to its set stable value. The pump-cell then pumps oxygen into the monitoring chamber by way of the reversed current in the ECM’s AFR pump-cell circuit. The ECM detects the reversed current and an injector pulsation-reduction command is issued bringing the mixture back to lean. Since the current in the pump cell circuit is also proportional to the oxygen concentration or deficiency in the exhaust, it serves as an index of the air/fuel ratio. The ECM is constantly monitoring the pump cell current circuitry, which it always tries to keep at a set voltage. For this reason, the techniques used to test and diagnose the regular Zirconium O2 sensor can not be used to test the wide-band AFR sensor. These sensors are current devices and do not have a cycling voltage waveform. The testing procedures, which will be discussed later, are quite different from the older O2 sensors.
atom
s either into, or out of, the diffusion chamber (monitoring chamber) which restores the monitoring chamber’s air/fuel ratio to stoichiometry. Although the ECM varies the current, it tries to maintain the pump circuit at a constant voltage potential.
as a scanner PID
. This is why the best way to test an AFR sensor’s signal is by monitoring the voltage conversion circuitry, which the ECM sends out as an AFR-voltage PID. It is possible to monitor the actual AFR sensor varying current, but the changes are very small (in the low milliamp range) and difficult to monitor. A second drawback to a manual AFR current test is that the signal wire has to be cut or broken to connect the ammeter
in series
with the pump circuit. Today’s average clamp-on ammeter is not accurate enough at such a small scale. For this reason, the easiest (but not the only) way to test an AFR sensor is with the scanner.
By using a scanner to communicate with the ECM, one can view AFR sensor activity. This data is typically displayed as WRAF, A/F, or AFR sensor voltage. However, on some vehicles and scanners it will show up as "lambda" or "equivalence ratio." If the PID displays a voltage reading, it should be equal to the sensor's reference voltage when the air/fuel mixture is ideal. The reference voltage varies from car to car, but is often 3.3v or 2.6v. When the fuel mixture becomes richer (on a sudden, quick acceleration), the voltage should decrease. Under lean conditions (such as deceleration) the voltage should increase.
If the scanner PID displays a "lambda" or "equivalence ratio," the reading should be 1.0 under stoichiometric conditions. Numbers above 1.0 indicate a lean condition while numbers below 1.0 indicate rich mixtures. The ECM uses the information from the sensors to adjust the amount of fuel being injected into the engine, so corresponding changes in the short-term fuel trim PID(s) should also be seen. Lean mixture readings from the AFR sensor will prompt the ECM to add fuel, which will manifest itself as a positive (or more positive) short-term fuel trim percentage.
Some technicians will force the engine to run lean by creating a vacuum leak downstream from the mass airflow sensor, and then watch scanner PIDs for a response. The engine can be forced rich by adding a metered amount of propane to the incoming airflow. In either case, if the sensor does not respond, it likely has a problem. However, these tests do not rule out other circuitry problems or ECM issues. Because an AFR sensor can be relatively expensive (up to $400 U.S dollars), a professional diagnosis is recommended.
above 1200°F (600°C). On these units the temperature is very critical and for this reason a special pulse-width
controlled heater circuit is employed to control the heater temperature precisely. The ECM controls the heater circuit.
s) at 14.7:1 air-fuel ratio. In this case a light negative hump is seen on the ammeter with the reading returning to 0.00 almost immediately. The fuel correction happens very quickly.
Air-fuel ratio
Air–fuel ratio is the mass ratio of air to fuel present in an internal combustion engine. If exactly enough air is provided to completely burn all of the fuel, the ratio is known as the stoichiometric mixture, often abbreviated to stoich...
sensor that is slowly replacing the Zirconium
Zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...
oxygen sensor
Oxygen sensor
An oxygen sensor, or lambda sensor, is an electronic device that measures the proportion of oxygen in the gas or liquid being analyzed. It was developed by the Robert Bosch GmbH company during the late 1960s under the supervision of Dr. Günter Bauman...
(O2 sensor) in modern motor vehicles.
Zirconia oxygen sensor
The early introduction of the oxygen sensor came about in the late 1970s. Since then Zirconia has been the material of choice for its construction. The Zirconia O2 sensor produces its own voltageVoltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
, which makes it a type of generator. The generated varying voltage shows up on the scope
Oscilloscope
An oscilloscope is a type of electronic test instrument that allows observation of constantly varying signal voltages, usually as a two-dimensional graph of one or more electrical potential differences using the vertical or 'Y' axis, plotted as a function of time,...
as the familiar 1 Hz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
sine wave
Sine wave
The sine wave or sinusoid is a mathematical function that describes a smooth repetitive oscillation. It occurs often in pure mathematics, as well as physics, signal processing, electrical engineering and many other fields...
, when in closed loop control. The actual voltage that is generated is the difference between the O2 content of the exhaust and that of the surrounding ambient air. The stoichiometric air-fuel ratio
Air-fuel ratio
Air–fuel ratio is the mass ratio of air to fuel present in an internal combustion engine. If exactly enough air is provided to completely burn all of the fuel, the ratio is known as the stoichiometric mixture, often abbreviated to stoich...
or the mixture of air-to-fuel equal to 14.7:1 is theoretically the best mixture ratio for gasoline engines.
This is the theoretical air -to- fuel ratio at which all of the fuel will react with all of the available oxygen resulting in complete combustion. At or near this ratio, the combustion
Combustion
Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...
process produces the best balance between power and low emissions. At a stoichiometric air-fuel ratio (14.7:1), the generated O2 sensor voltage is about 450 mV. The Engine Control Module (ECM) recognizes a rich condition above the 450 mV level, and a lean condition below it, but does not detect the extent of the richness or leanness. It is for this reason that the Zirconium O2 sensor is called a “narrow-band” O2 sensor.
Titanium oxygen sensor
The TitaniumTitanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
O2 sensor was used throughout the late 1980s and early 1990s on a limited basis. This sensor’s semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
construction makes its operation different from that of the Zirconium O2 sensor. Instead of generating its own voltage, the Titanium O2 sensor’s electrical resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
changes according to the exhaust oxygen content. When the air/fuel ratio is rich, the resistance of the sensor is around 950 Ohm
Ohm
The ohm is the SI unit of electrical resistance, named after German physicist Georg Simon Ohm.- Definition :The ohm is defined as a resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere,...
s and more than 21 Kilohms when the mixture is lean. As with the Zirconium sensor, the Titanium O2 sensor is also considered a narrow-band O2 sensor.
Narrow-band sensor
As mentioned before, the main problem with any narrow-band O2 sensor is that the ECM only detects that the mixture is slightly richer or leaner than 14.7:1. The ECM does not measure the operating air-fuel ratio outside the stoichiometric range. In effect it only detects that the mixture is richer or leaner than stoichiometry. An O2 sensor voltage that goes lower than 450 mV will cause a widening of injector pulse and vice-versa. The resulting changing or cycling fuel control (closed-loop) O2 signal is what the technician sees on the scope when probing at the O2 sensor signal wire.Modern sensors
The newer “wide-band” O2 sensor solves the narrow sensing problem of the previous Zirconium sensors. These sensors are often called by different names such as, continuous lambdaLambda
Lambda is the 11th letter of the Greek alphabet. In the system of Greek numerals lambda has a value of 30. Lambda is related to the Phoenician letter Lamed . Letters in other alphabets that stemmed from lambda include the Roman L and the Cyrillic letter El...
sensors (lambda representing air-fuel ratio
Air-fuel ratio
Air–fuel ratio is the mass ratio of air to fuel present in an internal combustion engine. If exactly enough air is provided to completely burn all of the fuel, the ratio is known as the stoichiometric mixture, often abbreviated to stoich...
), AFR (air-fuel ratio sensors), LAF (lean air-fuel sensor) and wide-band O2 sensor. Regardless of the name, the principle is the same, which is to put the ECM in a better position to control the air/fuel mixture. In effect, the wide-band O2 sensor can detect the exhaust’s O2 content way below or above the perfect 14.7:1 air/fuel ratio. Such control is needed on new lean burn
Lean burn
Lean burn refers to the use of lean mixtures in an internal combustion engine. The air-fuel ratios can be as high as 65:1, so the mixture has considerably less fuel in comparison to the stoichiometric combustion ratio ....
ing engines with extremely low emission output levels. The tighter emission regulations are actually driving this newer fuel control technology and in the process making the systems much more complex and difficult to diagnose.
Construction and operation
The wide-band O2 sensor looks similar in appearance to the regular Zirconium O2 sensor. Its inner construction and operation are totally different, however. The Wide-band O2 sensor is composed of a dual inner layer called “Reference cell” and “Pump cell”. The ECM’s AFR sensor circuitry always tries to keep a perfect air/fuel ratio (14.7:1) inside a special monitoring chamber (Diffusion Chamber or pump-cell circuit) by way of controlling its current. The AFR sensor uses dedicated electronic circuitry to set a pumping current in the sensor’s pump cell. In other words, if the air/fuel mixture is lean, the pump cell circuit voltage momentarily goes low and the ECM immediately regulates the current going through it in order to maintain a set voltage value or stoichiometric ratio inside the diffusion chamber. The pump cell then discharges the excess oxygen through the diffusion gap by means of the current created in the pump-cell circuit. The ECM senses the current and widens injector pulsation accordingly to add fuel.If on the other hand the air/fuel mixture goes rich, the pump cell circuit voltage rapidly climbs high and the ECM immediately reverses the current polarity to readjust the pump cell circuit voltage to its set stable value. The pump-cell then pumps oxygen into the monitoring chamber by way of the reversed current in the ECM’s AFR pump-cell circuit. The ECM detects the reversed current and an injector pulsation-reduction command is issued bringing the mixture back to lean. Since the current in the pump cell circuit is also proportional to the oxygen concentration or deficiency in the exhaust, it serves as an index of the air/fuel ratio. The ECM is constantly monitoring the pump cell current circuitry, which it always tries to keep at a set voltage. For this reason, the techniques used to test and diagnose the regular Zirconium O2 sensor can not be used to test the wide-band AFR sensor. These sensors are current devices and do not have a cycling voltage waveform. The testing procedures, which will be discussed later, are quite different from the older O2 sensors.
Comparison with mass airflow sensor
The AFR sensor operation can be thought of as being similar to the hot wire Mass airflow sensor (MAF). But, instead of an MAF hot wire, the ECM tries to keep a perfectly stoichiometric air/fuel ratio inside the monitoring chamber by varying the pump cell circuit current. The sensing part, at the tip of the sensor, is always held at a constant voltage (depending on manufacturer). If the mixture goes rich, the ECM will adjust the current flowing through the sensing tip or pump cell circuit until the constant operating voltage level is achieved again. The voltage change happens very fast. The current through the pump circuit also pushes along the oxygenOxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s either into, or out of, the diffusion chamber (monitoring chamber) which restores the monitoring chamber’s air/fuel ratio to stoichiometry. Although the ECM varies the current, it tries to maintain the pump circuit at a constant voltage potential.
Testing
As the ECM monitors the varying current, a special circuit (also inside the PCM) converts the current into a voltage value and passes it on to the serial data streamSerial communication
In telecommunication and computer science, serial communication is the process of sending data one bit at a time, sequentially, over a communication channel or computer bus. This is in contrast to parallel communication, where several bits are sent as a whole, on a link with several parallel channels...
as a scanner PID
PID controller
A proportional–integral–derivative controller is a generic control loop feedback mechanism widely used in industrial control systems – a PID is the most commonly used feedback controller. A PID controller calculates an "error" value as the difference between a measured process variable and a...
. This is why the best way to test an AFR sensor’s signal is by monitoring the voltage conversion circuitry, which the ECM sends out as an AFR-voltage PID. It is possible to monitor the actual AFR sensor varying current, but the changes are very small (in the low milliamp range) and difficult to monitor. A second drawback to a manual AFR current test is that the signal wire has to be cut or broken to connect the ammeter
Ammeter
An ammeter is a measuring instrument used to measure the electric current in a circuit. Electric currents are measured in amperes , hence the name. Instruments used to measure smaller currents, in the milliampere or microampere range, are designated as milliammeters or microammeters...
in series
Series and parallel circuits
Components of an electrical circuit or electronic circuit can be connected in many different ways. The two simplest of these are called series and parallel and occur very frequently. Components connected in series are connected along a single path, so the same current flows through all of the...
with the pump circuit. Today’s average clamp-on ammeter is not accurate enough at such a small scale. For this reason, the easiest (but not the only) way to test an AFR sensor is with the scanner.
By using a scanner to communicate with the ECM, one can view AFR sensor activity. This data is typically displayed as WRAF, A/F, or AFR sensor voltage. However, on some vehicles and scanners it will show up as "lambda" or "equivalence ratio." If the PID displays a voltage reading, it should be equal to the sensor's reference voltage when the air/fuel mixture is ideal. The reference voltage varies from car to car, but is often 3.3v or 2.6v. When the fuel mixture becomes richer (on a sudden, quick acceleration), the voltage should decrease. Under lean conditions (such as deceleration) the voltage should increase.
If the scanner PID displays a "lambda" or "equivalence ratio," the reading should be 1.0 under stoichiometric conditions. Numbers above 1.0 indicate a lean condition while numbers below 1.0 indicate rich mixtures. The ECM uses the information from the sensors to adjust the amount of fuel being injected into the engine, so corresponding changes in the short-term fuel trim PID(s) should also be seen. Lean mixture readings from the AFR sensor will prompt the ECM to add fuel, which will manifest itself as a positive (or more positive) short-term fuel trim percentage.
Some technicians will force the engine to run lean by creating a vacuum leak downstream from the mass airflow sensor, and then watch scanner PIDs for a response. The engine can be forced rich by adding a metered amount of propane to the incoming airflow. In either case, if the sensor does not respond, it likely has a problem. However, these tests do not rule out other circuitry problems or ECM issues. Because an AFR sensor can be relatively expensive (up to $400 U.S dollars), a professional diagnosis is recommended.
Operating temperature
Another major difference between the wide-band AFR sensor and a Zirconium O2 sensor is that it has an operating temperatureOperating temperature
An operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the...
above 1200°F (600°C). On these units the temperature is very critical and for this reason a special pulse-width
Pulse-width modulation
Pulse-width modulation , or pulse-duration modulation , is a commonly used technique for controlling power to inertial electrical devices, made practical by modern electronic power switches....
controlled heater circuit is employed to control the heater temperature precisely. The ECM controls the heater circuit.
Advantages
The wide operating range coupled with the inherent fast acting operation of the AFR sensor puts the system always at stoichiometry, which reduces a great deal of emissions. With this type of fuel control, the air/fuel ratio is always hovering close to 14.7:1. If the mixture goes slightly rich the ECM adjusts the pump circuit’s current to maintain the set operating voltage. The current is detected by the ECM’s detection circuit, with the result of a command for a reduction in injector pulsation being issued. As soon as the air-fuel mixture changes back to stoichiometry, because of the reduction in injector pulsation, the ECM will adjust the current respectively. The end result is no current (0.00 ampereAmpere
The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...
s) at 14.7:1 air-fuel ratio. In this case a light negative hump is seen on the ammeter with the reading returning to 0.00 almost immediately. The fuel correction happens very quickly.