Oxygen sensor versus air fuel ratio sensor? (Solved)


  • What are the biggest difference between an Oxygen sensor and an air fuel ratio sensor? 1. Both measure oxygen in the exhaust stream, but a wide range air fuel ratio sensor (AFR) measures the actual amount of oxygen, as opposed to an ordinary oxygen sensor that just reports a rich or lean condition in the exhaust.

How do I know if my air fuel ratio sensor is bad?

Symptoms of a defective Oxygen/Air-Fuel Ratio Sensor: Common indications of a bad oxygen/air-fuel ratio sensor include rough idling, engine pinging, poor gas mileage and increased exhaust emissions. One of the first symptoms of a faulty sensor is the lighting up of the “Check Engine” light.

What is a fuel ratio sensor?

The air fuel ratio sensor is typically in the exhaust manifold or in the front exhaust pipe. It measures the oxygen in the exhaust and sends that information to the ECU. The ECU, based on the air-to-fuel ratio, adjusts the blend to keep it at the prime level. This level is generally 14.7:1.

What does a bad air fuel ratio sensor do?

The air-fuel ratio sensor monitors the oxygen content in the exhaust stream and sends data to the computer so that it can add or subtract fuel. If there is any problem with the sensor, it can send a bad or false signal to the computer which can throw off its calculations and cause it to waste fuel excessively.

Is a NOX sensor the same as an O2 sensor?

Both the oxygen sensor and the nox sensor are mounted on the exhaust pipe, and the sensors look similar. But nox sensor directly transmits the data to ECU,the nox sensor datta is more accurate,and in the future nox sensor will replace the oxygen sensor step by step.

Is air fuel sensor same as oxygen sensor?

An air/fuel sensor can read a much wider and leaner range of fuel mixtures than a conventional O2 sensor. That’s why they’re also called “ wideband” O2 sensors. An A/F sensor, by comparison, produces a changing current signal that varies in direct proportion to the amount of unburned oxygen in the exhaust.

Can a bad downstream oxygen sensor cause a rough idle?

Symptoms of a Bad Oxygen Sensor Sensors simply report information. The downstream or diagnostic sensors only monitor the exhaust leaving the catalytic converter and will not cause such an issue. Other symptoms of a bad oxygen sensor include a rough idle, a misfire, and/ or hesitation when trying to accelerate.

Will a oxygen sensor stop a car from running?

Driving with a faulty O2 sensor means the computer won’t be getting the correct reading of the mixture and hence it won’t be able to adjust the air-fuel mixture properly. But if your engine starts and runs, and can stay running, it’s drivable.

What is the purpose of a O2 sensor?

The oxygen sensor of your car measures the amount of oxygen in the exhaust gasses that exit the engine. It sends real-time data about the amount of unburnt oxygen in the exhaust system to the engine’s computer to determine the correct air-to-fuel ratio for the car’s engine.

What is an O2 sensor do?

What O2 Sensors Do. Put simply, O2 sensors help regulate your car engine’s air-fuel ratio. Each explosion in your car’s combustion chambers is caused by an ignition of air and fuel. When the ratio of air and fuel taken into those chambers remains optimal, your car will run and idle just fine.

What are the symptoms of a faulty oxygen sensor?

Here are some of the most common signs that your oxygen sensor is bad.

  • A Glowing Check Engine Light. The bright orange Check Engine light in your dashboard will usually glow if you have a bad oxygen sensor.
  • Bad Gas Mileage.
  • An Engine That Sounds Rough.
  • An Emissions Test Failure.
  • An Older Vehicle.

Can a bad air fuel ratio sensor cause a no start?

Without the correct ratio, the engine won’t run. If it is too dirty, it will be unable to correctly measure the air fuel ratio, and cause your car to not start or exhibit other unusual symptoms. Preventing engine damage is another reason that a sensor fault can cause a hard to start problem.

Should I replace all O2 sensors at once?

Manufacturers recommend replacing O2 sensors in pairs (both Upstream or both Downstream). An older, slower sensor can cause an imbalance in the engine management system, leading to poor fuel economy and possible damage to the catalytic converter.

What’s the difference between downstream and upstream oxygen sensors?

The upstream sensor monitors the level of pollutants in the engine’s exhaust and sends this information to the ECU that continuously adjusts the air-fuel ratio. The downstream sensor measures the level of pollutants passing through the catalytic converter.

Can you drive with a bad NOx sensor?

In conclusion, you can drive with a bad knock sensor —that is, if you want to destroy your engine and get atrocious performance from your car. The moment you confirm that your knock sensor has seen better days, it would be wise to swap it right away with a high-quality replacement.

Air Fuel Sensor Versus the Oxygen Sensor

The electronic control unit, often known as the ECU, was first created in the 1970’s. Since then, there have been several developments and technical advancements in the automobile manufacturing industry. Most modern automobiles are equipped with an electronic control unit (ECU) that communicates with sensors located throughout the vehicle. The majority of automobiles also use the same sort of sensors, which are just manufactured by different automobile manufacturers for their vehicles. When you own a Lexus, it is critical to have the vehicle’s owner’s handbook on hand and to be familiar with the sensors installed in your vehicle and how each sensor functions.

Visit drivers.Lexus.com and input your car model and year in the “Select A Vehicle” box at top of page, which is labeled “Select A Vehicle.” This will provide you with your Lexus handbook.

One of the sensors that you should become familiar with is the air fuel ratio sensor, as well as the oxygen sensor.

What Is The Air Fuel Ratio Sensor

Cars have had oxygen sensors since the 1980s, but it wasn’t until the early 2000s that more precise sensors were created, resulting in the development of air fuel ratio sensors (also known as A/F sensors). The A/F sensor measures the amount of oxygen present in the car’s exhaust in a more comprehensive and effective manner than the previous sensor. Despite the fact that it has a similar appearance to the oxygen sensor, it has more exposed wires. The A/F can detect a far wider and leaner range of fuel mixes than the previous model.

Air fuel ratio sensors are often found in the exhaust manifold or the front exhaust pipe of a vehicle.

Based on the air-to-fuel ratio, the engine control unit (ECU) changes the mix to keep it at the optimal level.

It is critical that the A/F sensor remains in good working order in order for your automobile to continue to perform at its peak performance level.

What is the Oxygen Sensor?

The oxygen sensor (O2) is a critical component of your vehicle’s emissions system, and has been since the beginning of the usage of electronic control units. When the O2 sensors were developed, they were designed to keep track of how much oxygen is present in your vehicle’s exhaust stream while also testing for efficiency and certifying that the catalytic converter is in perfect working order. Whether the air/fuel combination is too lean or too rich is determined by the sensor. That information is passed back to the ECU, which changes the metering and timing of the fuel to ensure that your automobile is running on the appropriate blend of gasoline and oxygen.

Because your vehicle has two sensors that monitor the exhaust, the O2 sensor will be located on the opposite side of the catalytic converter from the A/F sensor and vice versa.

Why is it Necessary to Have Both Sensors?

Newer automobiles, such as your Lexus or luxury vehicle, are equipped with both sensors. One is located “downstream” of the converter, while the other is located “upstream” of the converter. One of the main reasons for using both sensors is to properly calibrate the information that is received by the ECU. In a nutshell, two interpretations are preferable to one. These readings will provide you with the information you need to ensure that your vehicle is operating at peak performance, which will have an impact on a variety of factors, including the quality of your ride and your fuel economy.

If you have any queries about the air fuel ratio sensor or the oxygen sensor in your Lexus, please contact Earnhardt Lexus or our service department to find out more information about them.

Oxygen Sensors vs. Air Fuel Ratio Sensors

When it comes to oxygen sensors in modern automobiles, there are two types to choose from: traditional oxygen sensors and the newer air/fuel sensors (also called a wide range or wideband oxygen sensor). If you want to understand more about how these two systems operate in the vehicles in your shop, Master Technicians Mark Kenyon and Mark Ingram of Garage Gurus will go through each system in depth. After seeing this video, you will be able to distinguish between the distinctions between the oxygen sensor and the air fuel sensor, as well as the effects that each has on long- and short-term fuel trimming.

You can rely on Garage Gurus to assist you in completing your customer’s automobile repair correctly.

The latest in automotive training is available to you locally, online, or at one of our automotive training locations.

Check out these videos from the professionals at Garage Gurus if you want to learn more more about engine repair:

  • The Ignition Coil Test for Coil-On-Plug Systems, as well as the Power Steering Pressure Test, are all performed.

Garage Gurus’ Master Technicians are offering the following training courses to help you take your profession to the next level:

  • The Wide Range Air System includes the following components: fuel sensor diagnostics
  • Mass air flow volumetric efficiency testing on today’s engines
  • Engine performance and driveability
  • And engine performance and driveability.

Oxygen or Air Fuel Ratio Sensor

The sensor that monitors the quantity of oxygen in the exhaust gases of today’s current vehicles is a vital component of the car of the future. This sensor collaborates with the vehicle’s Electronic Control Unit (ECU) to ensure that the engine’s operating efficiency is maintained at all times. As a result, if the quantity of oxygen in the exhaust stream is too high (a lean mixture), the sensor is able to relay this information back to the ECU, which changes the amount of fuel delivered to each cylinder in order to maintain the ideal air/fuel ratio.

  1. It is critical to determine the type of sensor before proceeding with the repair.
  2. In order to detect whether the current air/fuel ratio is richer or leaner than the stoichiometric air/fuel ratio, the ECU utilizes the output voltage to calculate the air/fuel ratio (14.7:1).
  3. In order to provide a reaction that is directly proportionate to the input of the current air/fuel ratio in the exhaust system, changes in output current are converted into voltage by the ECU.
  4. To provide an analogy, imagine that the oxygen sensor is connected to a light bulb and delivers electricity to the vehicle’s ECU, which serves as an on/off switch, turning the bulb on and off.
  5. In order to identify whether the engine is running rich or lean, a narrow-band oxygen sensor is used, which does not measure the extent to which the engine is rich or lean.
  6. This makes the air-fuel ratio sensor superior to oxygen sensors in current autos, as seen in the chart below.
  7. Rough idling, engine pinging, poor gas mileage, and increased exhaust emissions are all signs of a faulty oxygen/air-fuel ratio sensor, according to the manufacturer.
  8. Buy Auto Parts provides Oxygen Sensors for sale online!
  9. Once you have selected the correct year, make, and model of your car, we will ship you the oxygen sensor.
  10. In addition, we provide free delivery on orders over $99.

If you are having difficulty identifying your component, our customer service team is here to assist you: phone us at or send us an email at Please have a look at our large selection of well tested OEM replacement and aftermarket parts for every make and model.

AF sensor vs O2 sensor

The sensor that monitors the quantity of oxygen in the exhaust gases of today’s contemporary vehicles is a key component of the vehicle. Combined with the vehicle’s Electronic Control Unit (ECU), this sensor ensures the engine’s operating efficiency is maintained on a continuous basis. Depending on whether the oxygen content in the exhaust gas is too high (a lean mixture) or too low (a rich mixture), the sensor is able to transmit this information back to the ECU, which in turn adjusts the amount of fuel that is delivered to the cylinders in order to maintain the ideal air to fuel ratio.

  1. The type of sensor must be determined before any repairs can be performed.
  2. A steady voltage of around 0.4V is provided to the air-fuel ratio sensor in vehicles equipped with such a sensor, which produces an output current that fluctuates in response to the amount of oxygen present in the exhaust stream.
  3. A comparison of oxygen sensors and air-fuel sensors is shown below.
  4. To provide an analogy, imagine that the oxygen sensor is connected to a light bulb and delivers electricity to the vehicle’s ECU, which works as an on/off switch, switching the light on and off.
  5. b)A narrow-band oxygen sensor indicates whether the engine is running rich or lean, but it does not measure the amount to which the engine is running rich or lean.
  6. As a result, in current autos, the air-fuel ratio sensor is preferred over the oxygen sensor.
  7. Ridiculous starting, pinging engines, poor gas mileage, and higher exhaust emissions are all signs of a malfunctioning oxygen/air-fuel ratio sensor, according to the EPA.
  8. Buy Auto Parts provides Oxygen Sensors for sale.
  9. Once you have selected the correct year, make, and model of your vehicle, we will ship you the oxygen sensor.
  10. Purchases above $99 are eligible for free delivery.

Please contact our customer service team if you are having difficulty identifying your part: phone us at or send an email to. Please feel free to browse through our vast inventory of well tested OEM replacement and aftermarket parts for every make and model.

O2 Sensor | Oxygen Sensor

The interior structure of the oxygen sensor is seen in Figure 2. In the illustration, an oxygen sensor is created from two platinum electrodes with a zirconium dioxide element sandwiched in between the electrodes. As exhaust displaces oxygen in the oxygen pipe, the oxygen pipe becomes clogged. In response to the potential difference in oxygen concentration between the outside air and the inside of an exhaust pipe, the platinum electrodes respond. Electrical current is generated as a result of this potential difference (i.e.

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An O2 sensor typically generates between 0.2 and 0.7 volts while operating at 600 degrees Fahrenheit.

It will produce more than 0.5V for a rich mixture and less than 0.5V for a lean combination.

AF Sensor | Air Fuel Ratio Sensor | Air Fuel Sensor

Air Fuel Ratio Sensor Inside Structure: Figure 3 displays the internal construction of the Air Fuel Ratio Sensor. The functioning of this sensor is based on that of the original oxygen sensor. Essentially, it is made up of two oxygen sensors that are located in a single space with a common chamber between them. The diffusion chamber and the air reference chamber are two chambers that have been added to the basic zirconium sensor in this wideband Air Fuel sensor. Conclusion: Voltage is used to measure oxygen in the O2 Sensor.

The operation of the AF sensor necessitates the usage of very little electricity.

What is difference between or comparison between

The differences and comparisons between various equipment and phrases are discussed in the following links: A comparison between SPI and I2C is shown. What is the difference between PXI and PCI? Microscope vs. Telescope: Which Is Better? Comparison between Amplitude Modulation and Angle Modulation what is the difference between a modem and a router Sensors for measuring the air-fuel ratio versus oxygen sensors Radiometer versus Spectrometer vs Spectroradiometer vs Spectroradiometer The difference between a clamp meter and a digital multimeter Colorimeter versus Spectrophotometer: Which Is Better?

The difference between Venturi meter and Orifice meter is the difference between Lux and Lumens.

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September 13, 2021 is the most recent update. Traditional oxygen sensors were phased out in the early 2000s, and were replaced by more precise air fuel ratio monitors, albeit they are still referred to as ‘oxygen sensors’ or “O2 sensors.” Sensor for measuring the air-to-fuel (A/F) ratio The air fuel ratio (A/F) sensor detects the amount of oxygen in the exhaust across a broader range of temperatures. The “broadband lambda sensor” or “lambda probe” is another name for this device. This sensor is located either in the exhaust manifold or the front exhaust pipe, immediately before the catalytic converter.

The oxygen concentration in the exhaust is measured by the air fuel ratio sensor, which then provides input to the engine computer (PCM).

The computer changes the air-to-fuel ratio based on the signal from the air-to-fuel ratio sensor to ensure that it remains at the optimal level, which is around 14.7:1, or 14.7 parts air to 1 part fuel.

Air fuel ratio sensor problems

Air fuel ratio sensors are prone to malfunctioning on a regular basis. Occasionally, a sensor becomes polluted or fails completely. In some vehicles, the heating element within the sensor might become inoperable, resulting in the malfunctioning sensor. When the code P0135 appears in many Toyota and Honda vehicles, for example, it may be caused by a faulty heating element within the sensor’s housing. The heating element of the A/F sensor is tested in this article: code P0135. To learn more, go here.

For example, in an earlier Mazda 3, the sensor wire may rub against the bracket and short out, resulting in the P0131 code being shown on the dashboard.

P0131, P0134, P0135, P0133, P0031, and P1135 are the most often seen OBDII fault codes associated with an air fuel ratio sensor.

In some vehicles, you may notice a decrease in fuel efficiency or a decrease in driving stability.

Air fuel ratio sensor diagnostic

The diagnosis of an air fuel ratio sensor is carried out in accordance with the manufacturer’s troubleshooting protocol for the fault code that has been established in the vehicle. The initial step is to look for technical service bulletins that are relevant to the situation (TSBs). Damage, corrosion, loose pins, and other issues with the sensor’s wire and connector must be looked for and corrected. Checking the air fuel ratio sensor using a scan tool is a good practice. The sensor signal must next be examined with a scan tool, depending on the issue code that was received.

This air fuel sensor is in perfect working order.

In this instance, your mechanic may propose that you get the air fuel ratio sensor replaced in order to avoid the chance of an intermittent failure.

Rear Oxygen Sensor

Diagram of the rear (downstream) oxygen sensor After the catalytic converter, an additional or downstream oxygen sensor is put in the exhaust system. It is used to determine the quantity of oxygen present in the exhaust gases that exit the catalytic converter. In order to monitor the effectiveness of the catalytic converter, the information from the rear oxygen sensor is collected. A continual comparison is made between the data from the front and rear oxygen sensors by the engine computer, also known as the PCM (see the diagram).

In the event that the catalytic converter fails, the PCM will illuminate the “Check Engine” light to alert you. It is possible to inspect the back oxygen sensor using a scan tool or a lab scope.

Air Fuel Ratio/Oxygen Sensor Identification

The front oxygen sensor, also known as the air fuel ratio sensor, is fitted prior to the catalytic converter; it is referred to as a ‘upstream’ or ‘Sensor 1’ in some circles. It is referred to as ‘downstream’ or Sensor 2 the rear oxygen sensor that is put after the catalytic converter. One bank of cylinders is found in a conventional in-line 4-cylinder engine (Bank 1). As a result, with an in-line 4-cylinder engine, the name “Bank 1, Sensor 1” simply refers to the oxygen sensor located in front of the engine.

  • There are two banks in a V6 or V8 engine (or two parts of that “V”).
  • Bank 1 and Bank 2 are defined differently by different automobile manufacturers.
  • As an example, according to the Toyota bulletinTSB-0398-09, bank 1 is located at the rear of the engine, whereas bank 2 is located at the front of the engine in the V6 Camry, Highlander, Avalon, Sienna, and Solara.
  • Bank 1 is located in the rear of a 2003 Nissan Maxima, and bank 2 is located at the front.

ReplacingAir Fuel Ratio/Oxygen Sensor

The front oxygen sensor, also known as the air fuel ratio sensor, is positioned before to the catalytic converter; it is referred to as a ‘upstream’ or ‘Sensor 1’ in the automotive industry. Sensor 2 is the name given to the rear oxygen sensor that is fitted after the catalytic converter. Only one bank of cylinders is found in a standard in-line 4-cylinder engine (Bank 1). The name “Bank 1, Sensor 1” merely refers to the front oxygen sensor of an in-line 4-cylinder engine, which is referred to as “Bank 1.” Specifically, the rear oxygen sensor is designated as “Bank 1, Sensor 2.” According to convention, Bank 1 refers to the engine bank that comprises cylinder 1.

In most cases, “Bank 1” is the name given to the bank that includes cylinder number 1.

For information on which bank 1 and bank 2 are in your automobile, consult a repair manual or search the internet using the year, make, model, and engine size of your car as keywords.

In a V6 Mazda 6 or V6 Mazda Tribute from 2003 to 2008, bank 1 is located at the rear of the vehicle, and bank 2 is located at the front. Bank 1 is located in the rear of a 2003 Nissan Maxima, whereas bank 2 is located at the front.

Easy Air-Fuel and Oxygen Sensor Diagnosis

The proprietor of Car Clinic, a cutting-edge auto repair business in Mahopac, New York, Truglia is a self-made millionaire. He holds an ASE A6 certification and an M.A. in English from Columbia University. Technicans Service Training and Automotive Technician Training Services have both provided him with formal training in the automotive industry. Located in the heart of downtown Los Angeles, Car Clinic’s facility is fully equipped with state-of-the-art factory-level equipment and maintains automobiles from all over the world, including diesels and hybrids.

  1. G.
  2. Some technicians, even those who have been in the industry for a long time, are still perplexed by how to diagnosis an air-fuel sensor or are unsure of what to look for while evaluating a rear oxygen sensor, despite their years of experience.
  3. Allow me to put the record straight: there are various methods by which you may diagnose any air-fuel or oxygen sensor and be certain that you are doing the proper repair.
  4. These are the fundamentals.
  5. The vehicle’s personal emissions analyzer is comprised of oxygen and air-fuel sensors.
  6. Before and after the catalytic converter, the air-fuel and oxygen sensors operate in concert to provide accurate data.
  7. We’ll get into diagnosing catalytic efficiency later on by looking at the rear oxygen sensor, but first let’s make sure we understand how oxygen and air fuel sensors govern fuel control on a vehicle’s fuel injection system.
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Because these sensors fail at a reasonably high rate, it is critical that we understand how they should function and what method we should follow when diagnosing them when they fail.

DTCs are nearly often caused by faulty sensors, which may be identified using the Ohms function on your meter.

A sensor that is clearly dead in the water and not providing any feedback is almost certainly not a wiring issue.

Oxygen sensors create their own voltage, and if they do not produce any voltage, they are clearly faulty.

You’ll see that it generates its own voltage.

Stick with the sensor from the original equipment manufacturer.

Ignore the parts salesman and simply purchase the appropriate sensor.

Older American automobiles often feature Bosch engines, although the majority of newer vehicles have Denso engines.

Walker does not manufacture their own sensors, rather they rebox the original equipment sensor at an estimated 80 percent clip.

Often, you may purchase the original equipment manufacturer’s brand through the aftermarket, as long as you keep with the brand you removed from the car.

The quantity of oxygen in the exhaust that is used in the combustion process is measured by the oxygen sensor in the exhaust.

This indicates that the item is in EXCELLENT CONDITION.

This is indicative of a CLEAN CONDITION.

For post-catalytic converter oxygen sensors that are utilized for fuel regulation, the following is true: When working properly, post-cat oxygen sensors provide a consistent voltage that ranges between 500 and 700 mV.

On some cars, the rear sensor does have an impact on fuel regulation to a certain extent.

Unfortunately, there is no way to determine what the best post-cat oxygen sensor voltage is in a generalized fashion.

The following procedure can be used to test both the front and rear oxygen sensors: To ensure that the sensor responds appropriately to both rich and lean circumstances, just produce a vacuum leak to make the system run lean and inject some propane to make the system run rich to test the sensor’s response.

  1. Remember to pump the brakes a number of times once you’ve finished putting everything back together after you’ve finished.
  2. If this is the case, you may have a sensor that is “lazy” and needs to be replaced.
  3. Despite the fact that Mode 5 is a thing of the past, Modes 5 and 6 function in the exact same manner.
  4. Mode 5 is not available on all cars, with the exception of some pre-CAN automobiles, however when it is, it is recommended that you read the data.
  5. Making a choice on a P0420 DTC can be aided by the results of this investigation.
  6. When Mode 5 is not accessible, Mode 6 should be used to display the results of the oxygen sensor test.
  7. When the voltage of air-fuel sensors increases, it indicates a lean situation, and when the voltage decreases, it indicates a rich condition.

Every time, the post-cat sensor is a conventional oxygen sensor.

Learn everything you can about the air-fuel sensor.

* Don’t be fooled by scan tool PIDS.

You will require a scanning instrument that provides accurate improved data.

The air fuel sensor voltages in newer cars will be more accurate.* In typical OBD II, you’ll frequently see a portion of the real voltage shown.

For voltage levels that begin with 3.3 volts (such as Toyotas), displaying them correctly using the 0 to 1 voltage scale is quite difficult.

You must be familiar with the specifications of air-fuel sensors.

It is extremely difficult to troubleshoot an air-fuel sensor if you do not know what your PID value is supposed to be.

Toyota has a 3.3-volt alternator, Honda has a 2.8-volt alternator, Hyundai has a 1.9-volt alternator, Subaru has a 2.44-volt alternator, Nissan has a 1.47-volt alternator, and Lambda has a one-volt alternator (all European manufacturers).

Using the example, a Lambda of 0.85 may be used to set a system rich DTC with a long term forward time (LTFT) of – 15 percent.

In any case, you may connect your meter in series with the air-fuel sensor in amps mode instead of paralleling them.

Each milliamp over zero corresponds to a percentage point of leanness, while each milliamp below zero corresponds to a point of richness.

Identifying and diagnosing air-fuel sensors The air-fuel sensor may be evaluated in the same way as the oxygen sensor is, by putting the system into lean and rich conditions and checking that the sensor responds promptly and correctly.

You may compare the results with what you know to be acceptable.

The post-cat oxygen sensor that is used in conjunction with it should not oscillate, but should instead maintain a fairly constant voltage between 500 and 700 mV.

When the condition is favorable, they experience a drop in voltage.

This is the polar opposite of our natural tendency to interpret high voltages as a rich signal and low voltages as a lean indicator, so proceed with caution.

It is necessary to lean out the mixture in order to restore a correct air-fuel mixture when the engine speed and throttle position decrease, and the voltage rises as a result.

Because it is their role to do so, oxygen and air-fuel sensors should behave in a predictable fashion.

If the cat is functioning properly, it will remove pollutants from the atmosphere, and the sensors will relay this information to the PCM.

The air-fuel sensor, on the other hand, will be operating at a steady voltage.

If the catalytic converter is not functioning properly, the post-cat oxygen sensor will display the same reading as the post-cat oxygen sensor.

This is common during a sudden fuel incident that the catalytic converter, even if it is in good working order, is unable to clean up immediately.

The car was otherwise in good working order.

After that, we changed the oil and got the vehicle back on the road.

It was at this point that the fun began.

Following that, we sought for TSBs, but were unable to locate any, so we turned to Identifix for help.

It said that if the post-cat oxygen sensor was rich and the short term fuel trim was low, the sensor should be replaced.

Obviously, the STFT was completely out of whack and was suggestive of either an oxygen sensor that had been moved lean or a significant vacuum leak at the time.

This looked to be exactly what was taking place.

I guess that’s all there is to it.

As a result, we needed to determine whether or not the air-fuel sensor was operating within specifications.

Please excuse the poor quality of the photo, but these screen shots were made in real-world store circumstances.

We tried adding propane, but the sensor remained immobile.

The sensor began to function properly again a few minutes after we finished the test, and the Lambda value dropped to one hundred.

Our investigation revealed that an occasionally faulty air-fuel sensor had been discovered in the act.

Testing this sensor did not necessitate the use of complicated back probing or the consulting of a wiring diagram.

The voltage on our meter indicated 2.44 V.

The automobile was dispatched and has not been seen or heard from since.

They do little more than inform the PCM if the car is running rich or lean.

Air-fuel sensors, on the other hand, have been in use on numerous cars for more than a decade.

When you have the proper standards and testing methodologies in place, there should be no reason why you cannot diagnose these sensors quickly and easily using the information provided here.

Due to NASCAR’s decision to replace carburetion with fuel injection in the Sprint Cup series in 2012, Bosch will be the exclusive provider of oxygen sensors for the new engines.

These advanced sensors will offer critical information for the regulation of the fuel injection engine management systems in the race vehicles.

Because of the switch to fuel injection, NASCAR drivers will have greater control over their vehicles’ performance as well as their vehicle’s fuel usage.

What is the mechanism through which oxygen sensors perform this extremely vital function?

When an oxygen ion transfer takes place between “reference air” inside the cell and the outside environment (the exhaust stream), it is referred to as a “Nernst cell.” In response to this ion flow, the sensor produces detectable voltage, which corresponds to the difference in oxygen concentration between ambient air outside of the sensor and reference air within the sensor.

  1. Bosch’s groundbreaking automotive oxygen sensor was fitted for the first time in a Volvo in 1976, following years of investigation, testing, and engineering development.
  2. An oxygen sensor’s output voltage decreases when the mixture is too lean (higher than 14.7:1), whereas an oxygen sensor’s output voltage increases when the mixture is too rich (less than 14.7:1).
  3. In addition to the sensor itself, the kind of fuel delivery system that the engine is employing influences the reaction time of oxygen sensors to changes in the exhaust oxygen level.
  4. Throttle body fuel injection systems switch two or three times per second at 2,500 revs, whereas multipoint fuel injection systems switch five to seven times per second at the same speed.
  5. The very advanced Bosch heated wide-band oxygen sensor, which is employed by NASCAR, makes use of an internal multilayer ceramic strip and introduces a completely new idea — the “pumping cell” — to the equation.

This variable signal can be used to report readings ranging from very rich to very lean and anywhere in between.

How Air Fuel Ratio Sensors Work

However, although the air/fuel ratio sensor is not something you think about often, it is a vital component of your car’s pollution control system. Here’s everything you need to know about air/fuel ratio sensors.

Understanding Air/Fuel Ratio Sensors

An air/fuel ratio sensor is very similar to an oxygen sensor, often known as an O2 sensor, and is gradually replacing them in many cars. Particularly sensitive, turbocharged, and fuel-efficient engines are being developed nowadays. An air/fuel ratio sensor, in contrast to the susceptible oxygen sensor, has a large range rather than a small range, and it functions by conduction rather than generation, making it somewhat more accurate. Although air/fuel ratio sensors are not the same as oxygen sensors, they will be found on OBDII-compliant engines (Sensor 1 will always be on the exhaust manifold and Sensor 2 will always be after the catalytic converter) and will be referred to as O2 sensors in most OBDII literature, despite the fact that they are actually air/fuel ratio sensors.

  • Catering to the needs of today’s sensitive engines
  • Sensors with a wide dynamic range
  • Improved performance for turbocharged and fuel-efficient engines

Air/Fuel Ratio Sensor vs. O2 Sensor

Because of the two fundamental distinctions between how an air/fuel ratio sensor works and how an older oxygen sensor works, it makes all the difference in how they operate in terms of assisting the electronic control module (ECM or ECU) in tuning the engine’s performance. O2 sensors with a central core consisting of zirconium (or, in rare cases, titanium) generate voltage when oxygen molecules travel past them, as opposed to the newer O2 sensors. The new air/fuel ratio sensor controls current flow with the help of a dual core processor and specialised electrical circuitry.

  • The air is drawn into the first of the dual cells, which is known as the pump cell or diffusion chamber, from the exhaust stream.
  • The second compartment, known as the reference cell, contains outside air that is used as a reference in the calculations performed by the ECM (engine control module).
  • It also makes it possible to make adjustments more quickly.
  • The air/fuel ratio may also be changed by adjusting the current, which can even be reversed in polarity, in order to’suck in’ or ‘expel’ extra oxygen from the Pump Cell in order to make room for fresh measurements to be taken.
  • It is difficult to do diagnostics on the air/fuel ratio sensor because the changes in voltage that it monitors are so minute, on the order of milliamps.
  • Testing must be performed using a scanner designed specifically for this purpose or with a newer OBD scanner that has this feature.
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In order to properly calibrate the air/fuel ratio sensor on your vehicle, you must first determine the normal operating voltage for the sensor, which is typically in the volt range, as well as how your scanner will interpret the results, which could be in actual amp changes or in lambda ratios (starting at 1.0 as ideal).

Higher numbers correlate to a lack of information, while lower numbers correspond to a surplus of information, which corresponds to the conduction differential in the Pump Cell.

The Bottom Line on Air/Fuel Ratio Sensors

Manufacturers are turning toward air/fuel ratio sensors because they provide a faster and more controllable emissions management alternative, allowing for a more constant ideal air:fuel ratio (usually held at 14.7:1 in a gasoline engine). Whereas an earlier oxygen sensor may take more than a second to cause modifications and calibration for air:fuel ratio, an air/fuel ratio sensor may complete the task in fractions of a second, resulting in significant fuel savings.

Supersniffers: Why Air/Fuel Ratio Sensors?

When it comes to manufacturing autos, vehicle manufacturers encounter a slew of difficulties. One of the most difficult is striking a balance between the demand of consumers for high performance and the government’s obligation to keep the air pure. Numerous new technologies are being developed in response to the pressures placed on engineers to decrease tailpipe emissions, boost fuel economy, and improve engine performance. Systems for fuel and engine control management have advanced greatly over the years, and they are now entrusted with achieving that difficult balancing act.

  • ZrO2 sensors, such as the typical zirconia dioxide O 2 sensor, are frequently employed because they respond to variations in exhaust composition.
  • It also gives feedback to the powertrain control module (PCM) in order to maintain an air/fuel ratio that is close to stoichiometric (14.7:1) throughout the vehicle.
  • For almost 30 years, the zirconia dioxide oxygen sensor has remained essentially constant in its design.
  • The oxygen sensor must be operated at a temperature of around 650°F in order to function effectively.
  • The feedback loop for fuel management utilizing standard zirconia dioxide O 2 sensors is relatively small and frequently shifting, as seen in the diagram.
  • The PCM must be aware of the exact fuel mixture being used at any one moment in order to accomplish better fuel control management under any operating scenario.
  • Using air/fuel ratio sensors, closed-loop fuel management may be achieved even when the mixture is not stoichiometric.

Single-CellDual-Cell Air/Fuel Sensors

Wide-range, wide-band, linear, and lean are some of the terms used to describe air/fuel ratio sensors. Single-cell and dual-cell sensors are the two most common sensor designs. As with traditional oxygen sensors, single-cell sensors create voltage in a way similar to that of the sensor. They are utilized to manage current flow using a customized balanced control/monitoring circuit that is housed within the computer memory. Toyota makes use of a linear A/F ratio sensor with a single cell. Air/fuel sensors from Toyota and other manufacturers seem quite similar to the standard zirconia dioxide O2 sensors; both have four wires and look identical in terms of connections.

  1. Dual-cell sensors are often constructed in a planar configuration, and they are referred to as current-pumping or ion-pumping sensors in certain circles.
  2. Honda’s lean-burn engines make use of the lean A/F ratio sensor, which allows them to respond to extremely lean mixes (about 22:1) without sacrificing performance.
  3. In order to function correctly, both the single-cell and dual-cell sensors must be operated at higher temperatures (about 1200°F).
  4. The source of the ambiguity is the manner in which the sensor is reported by an OBD II generic scan tool.
  5. Toyota made the decision to report the air/fuel ratio sensor in a normalized voltage range of 0 to 1.0 volt in order to comply with the OBD II scan tool rules.
  6. 2 using a bar chart to describe its operation.
  7. This implies that the present air/fuel ratio is roughly 14.4:1, which indicates that the engine is somewhat rich.

The voltage displayed on the factory scan tool would be 3.2 volts in this instance.

If you weren’t aware that this car was equipped with an air/fuel ratio sensor, you would proceed with the standard zirconia sensor testing methods, which are described in detail here.

In this situation, though, the sensor would maintain a constant voltage of approximately.66 volts.

It is possible that the voltage might grow to approximately.8 volts if you introduced a vacuum leak into the system; however, the voltage would quickly drop back down to approximately.66 volts once the vacuum leak was eliminated.

The voltage would drop to approximately.48 volts if you used propane to create a rich state in your engine.

Most technicians would probably conclude that the oxygen sensor, and maybe even the PCM, should be replaced at this time.

Following that, Toyota corrected the problem in newer model automobiles.

If an A/F ratio sensor is employed, the characteristic may be presented in either amps or voltage, depending on the type of sensor utilized.

Figure 3 (on page 42) depicts numerous screenshots taken with the Mastertech scan tool.

The last line of the first screen capture reveals that the combination is being instructed to be 12 percent lean by the computer.

The center shot depicts the mixture after it has been instructed to be reduced to 0 percent.

The final capture reveals that the combination was 25 percent rich when the instruction was sent.

Based on this information, the A/F ratio sensor was found to be in good working order.

Following this, if no change in the air/fuel ratio sensor voltage is seen or if the sensor responds too slowly, the sensor heater should be tested to determine whether it is functioning properly.

The four-wire sensor is comprised of two wires for the heater and two wires for the signal from the sensor.


It is possible to test the sensor circuit with a low-current probe and a lab scope.

This is what you would expect to observe from a regular sensor on a lab scope, as seen in Fig.

Depending on the application, the heater circuit’s duty cycle is used to manage the current, and the peak amperage should be roughly 6 amps.

One method is using a voltmeter and examining each wire individually.

It is necessary to check for open or shorted circuits as well as probable PCM problems if the voltage at either wire is different from those figures.

The sensor will operate in the same manner as a normal O 2 sensor, with a voltage range of 0 to 1.0 volt.

The third method of inspecting the sensor signal circuit is attaching a DVOM in series with one of the signal wires in the sensor circuitry.

As the mixture swings, the current flow should change from positive to negative in direction.

It was discovered that the car had two reoccurring fault codes/DTCs: P0125 (Insufficient Coolant Temperature for Closed Loop Fuel Control) and P1135 (A/F Sensor Heater Circuit Malfunction, which was not listed here).

This code has absolutely nothing to do with the temperature of the engine coolant.

The following requirements must be satisfied in order for the monitor to function properly: Engine speed of 1500 rpm or more, vehicle speed of 25 to 62 mph, TPS must not display idle, these circumstances must last for at least 90 seconds, and the engine must run for at least 140 seconds are all required criteria.

The most typical reason for this is a malfunctioning sensor.

Mode 6 data can be used as a second method of determining the efficiency of a repair.

In this situation, the proper circumstances must be satisfied in order for the test to be successful. Following a road test, you’d need to double-check the Test Identification (TID) $07 and the Component Identification (CID) $0.01 records.

Current Pump Senors

In certain circles, the Bosch LSU 4 is referred to as a current pump sensor since it has a broad range of air/fuel ratio readings. One of the zirconia cells serves as an input to the PCM, and the other serves as a voltage regulator for the PCM. The pump cell functions as an ion pump, pushing oxygen ions from one side of the sensor to the opposite side of the sensor. The PCM continuously monitors the Nernst signal line and makes every effort to maintain the voltage at.45 volt. In order to maintain that voltage level, the PCM will increase and decrease the current flow to the pump cell on a continuous basis.

Its resistance ranges from 30 to 300 ohms and varies from sensor to sensor depending on the model.

Once again, using a scan tool is the most effective method of diagnosing this A/F ratio sensor.

The screen shots in Fig.

It will be necessary to do extra diagnostics if the air/fuel ratio parameter value displayed in the scan tool does not match the lambda value displayed on the gas analyzer.

10, a simplified Bosch LSU 4 sensor circuit is depicted.

According to the red numbers in Fig.

Inspect the heater circuit for damage.

The bare minimum current value varies depending on the service; consult the service literature for precise values.


Check the resistance of the trimming resistor between the input pump and output pump current wires with a digital multimeter (DVOM).


The voltage should be between 2.4 and 2.7 volts in most cases.

Inspect the pump cell and Nernst signal for proper operation.

After that, link one of the scope’s channels to the Nernst signal and the other channel to the input pump signal as shown below.

It is necessary to first examine the circuit, and then then to verify the input pump cell, if the voltage is less than 0.45 volts.

The scope must be set to bipolar or AC-coupled mode in order for you to be able to notice the negative swings in the voltage.

The ability to make a rapid and accurate diagnosis is dependent on having the appropriate tools for the job.

When it comes to air/fuel ratio sensors, the usage and refining of these devices will continue as long as the industry is driven by the need for more fuel economy, cleaner air, and higher performance. Obtain a PDF version of this document.

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