Oxygen Sensor
So you want to know a little more about how an oxygen sensor works? Well, as you may already know there are many sensors required for a modern engine to function, but arguably none are as important as the oxygen sensors.
These sensors read the amount of unburned oxygen in the exhaust. The computer then uses this reading to balance the fuel mixture. As oxygen content in the exhaust increases (known as a lean condition) the sensors voltage reading decreases.
This signals the computer to increase the amount of fuel the injectors are delivering. In turn the oxygen content in the exhaust decreases (known as a rich condition). The oxygen sensor voltage increases as a result of this richening, and the computer reacts by reducing fuel flow. As fuel quantity decreases we return to a lean mixture, and sensor voltage drops.
This signals the computer to increase the amount of fuel the injectors are delivering. In turn the oxygen content in the exhaust decreases (known as a rich condition). The oxygen sensor voltage increases as a result of this richening, and the computer reacts by reducing fuel flow. As fuel quantity decreases we return to a lean mixture, and sensor voltage drops.
This process repeats for as long as the engine is running. This continuous feedback loop is the heart of the fuel control system. Typical lean voltage readings are between 0 and .3 volts and rich readings range from .6 to 1 volt. An ideal fuel mixture (14.7:1) will produce a voltage of around .5 volts.
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So why not just maintain a constantly metered fuel amount that varies with throttle position? Well, many factors affect the amount of fuel required to maintain a 14.7:1 ratio.
Some of these factors include fuel quality, atmospheric pressure, humidity and more. Thus the need for O2 sensors! Sensor switch rates vary, but most modern sensors average a minimum of half a dozen switches per second.
The older sensors switched as slow as once per second, so you can imagine the improvement on emissions new style sensors have made!
Some of these factors include fuel quality, atmospheric pressure, humidity and more. Thus the need for O2 sensors! Sensor switch rates vary, but most modern sensors average a minimum of half a dozen switches per second.
The older sensors switched as slow as once per second, so you can imagine the improvement on emissions new style sensors have made!
Old style oxygen sensors
used before 1982 were of the 1 or 2 wire unheated type. These sensors would not actually begin to register a correct reading until the exhaust heated the sensor to its operating range.
This resulted in the computer running in "open loop" operation (using preset fuel values which actually run the engine rich) for longer periods of time. All newer style sensors are "Heated Oxygen Sensors" (HO2S) which incorporate a heating element used to bring the sensor up to operating temperature sooner, usually in less than a minute but as quickly as 10 seconds is possible!
The heating elements also prevent the sensors from cooling down when the engine is idling. These heated sensors are normally of the 3 and 4 wire design.
used before 1982 were of the 1 or 2 wire unheated type. These sensors would not actually begin to register a correct reading until the exhaust heated the sensor to its operating range.
This resulted in the computer running in "open loop" operation (using preset fuel values which actually run the engine rich) for longer periods of time. All newer style sensors are "Heated Oxygen Sensors" (HO2S) which incorporate a heating element used to bring the sensor up to operating temperature sooner, usually in less than a minute but as quickly as 10 seconds is possible!
The heating elements also prevent the sensors from cooling down when the engine is idling. These heated sensors are normally of the 3 and 4 wire design.
There are a few different style sensors, that vary by chemical make up and design but their purpose and function remains the same.
Oxygen sensors compare the oxygen content in the outside air to the oxygen content in the exhaust.
The outside air is brought into the sensor either through a vent in the sensor housing or through the wiring connector itself.
Some sensor types generate a voltage as oxygen content in the exhaust changes and some have a varying resistance.
Oxygen sensors compare the oxygen content in the outside air to the oxygen content in the exhaust.
The outside air is brought into the sensor either through a vent in the sensor housing or through the wiring connector itself.
Some sensor types generate a voltage as oxygen content in the exhaust changes and some have a varying resistance.
The newest style, Heated Wideband O2 Sensors,
have a voltage range of between 2 and 5 volts. Despite all of these differences, and actual readings produced by the sensors, the computer processes the information so that we have the expected 0 to 1 volt readings. There are a couple of exceptions, of course.
Some Heated Titania Type O2 Sensors can produce a voltage as high as 5 volts. This reading is not altered by the computer.
Another design of the same style sensor is configured to read values the opposite of what you'd expect. High voltages indicate a lean mixture and low voltages a rich mixture.
These 2 types of oxygen sensors are not common and were used mostly on a few Nissan, Jeep and Eagle applications. There's always got to be an exception right! Engineers, yeah I know!
have a voltage range of between 2 and 5 volts. Despite all of these differences, and actual readings produced by the sensors, the computer processes the information so that we have the expected 0 to 1 volt readings. There are a couple of exceptions, of course.
Some Heated Titania Type O2 Sensors can produce a voltage as high as 5 volts. This reading is not altered by the computer.
Another design of the same style sensor is configured to read values the opposite of what you'd expect. High voltages indicate a lean mixture and low voltages a rich mixture.
These 2 types of oxygen sensors are not common and were used mostly on a few Nissan, Jeep and Eagle applications. There's always got to be an exception right! Engineers, yeah I know!
You will also notice on most post '96 applications, there is a second set of oxygen sensors beyond the catalytic converters. These function the same as the front O2 sensors but their readings are used differently, and their purpose is to measure the efficiency of the converters, not monitor engine fuel ratios.
Oxygen Sensor Codes
Oxygen Sensors, otherwise referred to as O2 Sensors, and their related check engine light codes, can be troublesome at the very best!
It seems as though the check engine light and O2 Sensor codes have always gone hand in hand. I can't even begin to tell you how many perfectly good sensors I've seen replaced for a rich/lean code, or as a cure to a service engine light that comes and goes.
Fact is, when oxygen sensors were first being used, they did tend to fail. But manufacturers have been redesigning and tweaking these parts for over twenty years, and nowadays they are fairly bullet proof! Oxygen sensor codes no longer mean "replace the oxygen sensor", and this mindset can be costly! So what goes wrong during the diagnosis of these systems?
Fact is, when oxygen sensors were first being used, they did tend to fail. But manufacturers have been redesigning and tweaking these parts for over twenty years, and nowadays they are fairly bullet proof! Oxygen sensor codes no longer mean "replace the oxygen sensor", and this mindset can be costly! So what goes wrong during the diagnosis of these systems?
Nearly all engine control systems are comprised of multiple sub-systems. These sub-systems each depend on correct input from other areas of the electronic engine control system in order to perform their individual tasks properly. Confused yet? Keep reading, it'll make sense soon!
There are a large number of codes relating to oxygen sensor faults.
There are also a lot of faults that are often incorrectly thought to be caused by a bad oxygen sensor (also referred to as an O2 Sensor).
The O2 codes relating to circuit faults can be tested fairly easily. For help on these codes, check out our Generic OBDII list and definitely read our article on diagnosing the wiring related portion of a trouble code.
You will also run into codes for the O2 heater circuits.
Newer O2 sensors have heaters as part of their design. This allows them to get up to temperature quicker and begin functioning sooner, resulting in lower overall vehicle emissions.
The codes that may give you more of a run for your money are the PO171-P0175 generic (or similar manufacturer specific) codes. These codes tell you that despite the computers attempts to correct fuel trim (the amount of time the injectors stay on, delivering more or less fuel as needed); it can not obtain the correct air/fuel ratio.
These are known as rich and lean codes, and almost as often as I've seen new oxygen sensors installed for these codes, I've seen the same vehicles need re-diagnosis!
There are also a lot of faults that are often incorrectly thought to be caused by a bad oxygen sensor (also referred to as an O2 Sensor).
The O2 codes relating to circuit faults can be tested fairly easily. For help on these codes, check out our Generic OBDII list and definitely read our article on diagnosing the wiring related portion of a trouble code.
You will also run into codes for the O2 heater circuits.
Newer O2 sensors have heaters as part of their design. This allows them to get up to temperature quicker and begin functioning sooner, resulting in lower overall vehicle emissions.
The codes that may give you more of a run for your money are the PO171-P0175 generic (or similar manufacturer specific) codes. These codes tell you that despite the computers attempts to correct fuel trim (the amount of time the injectors stay on, delivering more or less fuel as needed); it can not obtain the correct air/fuel ratio.
These are known as rich and lean codes, and almost as often as I've seen new oxygen sensors installed for these codes, I've seen the same vehicles need re-diagnosis!
understand how an oxygen sensor works
The oxygen sensor screws into the exhaust and the sensor end protrudes into the pipe so that exhaust gases pass across the sensors internal element. There is a steel shielding with slots that direct exhaust flow across the actual element.
As a note, oxygen sensors used to determine engine fuel ratios are always located in front of the catalytic converter.
The sensors behind the cats are called monitors.
Sensors are normally described with the prefix HO2S meaning Heated Oxygen Sensor and followed with bank and position number.
For example
HO2S11 would be the oxygen sensor on bank 1 and first in line (pre catalytic converter) and HO2S12 would be on bank 1, but second in line (the monitor) An oxygen sensor creates a voltage between 0 and 1 volt by means of a chemical reaction between the sensor element and the oxygen in the exhaust passing across that element.
Outside air also passes through the sensor and it is a comparison between the oxygen content in the exhaust and fresh air that actually determines voltage output .
Believe it or not, the fresh air on many sensors actually travels through the wiring insulation! Anyway, a voltage closer to one volt indicates a rich condition and closer to zero indicates lean.
The ECU uses this valve to constantly adjust fuel trim to maintain a 14.7:1 air fuel ratio. We will be discuss fuel trim soon but this is a huge topic all on its own. Okay so this is where it gets somewhat complicated. Just kidding! It's all nuts and bolts; and wires, and chemicals and never mind!
Okay so the more oxygen in the exhaust (lean condition) the lower the voltage from the O2 sensor and the more fuel the computer calls for. Then the oxygen content in the exhaust drops (because it is being used up in the combustion process), and the voltage increases (rich condition) and the process repeats, for as long as the car is running, hundreds of times a minute. A gasoline internal combustion engine needs oxygen to burn the fuel. If the mixture is ideal (or 14.7:1) then all of the oxygen is consumed as the fuel is burned. The exact amount of fuel needed to produce a 14.7:1 air/fuel ratio varies with barometric pressure (altitude), relative humidity and fuel quality and condition; thus the need for oxygen sensors.
So knowing all of this what should we check when we have those pesky rich or lean codes?
The most common issues for lean codes are:
1. Vacuum leaks - check for failed or loose vacuum lines, leaking intake gaskets, intake air tubes loose or any other source of un-metered air leaks (leaks after the Mass Air Flow Sensor)
2. Restricted fuel filter or bent/pinched fuel system lines
3. Incorrect input from other sensors, such as the Mass Air Flow Sensor, which may not always drop a separate code
4. Engine misfire - Yes I know this one may seem weird. You might think that if there is a misfire then you will have all that unburned fuel and it should read rich; right? Well the O2 sensors read only oxygen content in the exhaust, so if you have all that unburned fuel from incomplete combustion then, you guessed it, you also have all that unburned oxygen. High O2 content in exhaust equals a lean reading!
There are also some other possibilities such as an internally leaking EGR system, (but this will typically set a separate code). A leak in the exhaust system before the O2 sensor will also cause incorrect readings. And always check for after- market modifications.
These can throw a wrench into the works! The only other possibilities (however unlikely), are wiring issues, computer concerns or a bad O2 sensor! There now that I've said it, on to rich codes.
The possible causes of rich codes are:
1. A leaking or faulty fuel injector
2. Fuel injector driver in computer shorted, or wiring short for injectors (likely a ground short)
3. Leaking or faulty fuel pressure regulator or restricted return line
4. Faulty evaporative emissions system - bleeding fuel vapors into engine (not commanded by computer)
5. On newer models a faulty fuel pump or fuel pump driver module
6. Faulty readings from other sensors such as a Mass Air Flow Sensor. You may actually be getting more air than the MAF tells the computer
7. Exhaust leaks before the sensor will cause erratic readings
8. After market components or performance chips
9. And yes, if I dare say it, possibly a computer, wiring issue or even a faulty O2 sensor!
The other codes we should address are those related to the sensors located after the catalytic converter. Though these may appear identical to the oxygen sensors pre-converter, they perform an entirely different task and are known as Monitors.
The only job of these sensors is to "monitor" the efficiency of the catalytic converters. The readings from these sensors should be much more stable and not fluctuating like the front O2 sensors.
The computer compares the readings from the oxygen sensors (pre cats) and the monitors (post cat) to determine if the catalytic converters are doing their job and "cleaning" the exhaust.
You never want to replace a monitor for a rich/lean concern as they have no bearing on these codes. As the converters begin to fail, you will see the monitors voltage readings follow the oxygen sensor readings.
Technically these are all "oxygen sensors" but it is important to distinguish the difference between pre-converter & post converter sensors, so I find it easiest to stick to calling the back ones monitors.
As a note, oxygen sensors used to determine engine fuel ratios are always located in front of the catalytic converter.
The sensors behind the cats are called monitors.
Sensors are normally described with the prefix HO2S meaning Heated Oxygen Sensor and followed with bank and position number.
For example
HO2S11 would be the oxygen sensor on bank 1 and first in line (pre catalytic converter) and HO2S12 would be on bank 1, but second in line (the monitor) An oxygen sensor creates a voltage between 0 and 1 volt by means of a chemical reaction between the sensor element and the oxygen in the exhaust passing across that element.
Outside air also passes through the sensor and it is a comparison between the oxygen content in the exhaust and fresh air that actually determines voltage output .
Believe it or not, the fresh air on many sensors actually travels through the wiring insulation! Anyway, a voltage closer to one volt indicates a rich condition and closer to zero indicates lean.
The ECU uses this valve to constantly adjust fuel trim to maintain a 14.7:1 air fuel ratio. We will be discuss fuel trim soon but this is a huge topic all on its own. Okay so this is where it gets somewhat complicated. Just kidding! It's all nuts and bolts; and wires, and chemicals and never mind!
Okay so the more oxygen in the exhaust (lean condition) the lower the voltage from the O2 sensor and the more fuel the computer calls for. Then the oxygen content in the exhaust drops (because it is being used up in the combustion process), and the voltage increases (rich condition) and the process repeats, for as long as the car is running, hundreds of times a minute. A gasoline internal combustion engine needs oxygen to burn the fuel. If the mixture is ideal (or 14.7:1) then all of the oxygen is consumed as the fuel is burned. The exact amount of fuel needed to produce a 14.7:1 air/fuel ratio varies with barometric pressure (altitude), relative humidity and fuel quality and condition; thus the need for oxygen sensors.
So knowing all of this what should we check when we have those pesky rich or lean codes?
The most common issues for lean codes are:
1. Vacuum leaks - check for failed or loose vacuum lines, leaking intake gaskets, intake air tubes loose or any other source of un-metered air leaks (leaks after the Mass Air Flow Sensor)
2. Restricted fuel filter or bent/pinched fuel system lines
3. Incorrect input from other sensors, such as the Mass Air Flow Sensor, which may not always drop a separate code
4. Engine misfire - Yes I know this one may seem weird. You might think that if there is a misfire then you will have all that unburned fuel and it should read rich; right? Well the O2 sensors read only oxygen content in the exhaust, so if you have all that unburned fuel from incomplete combustion then, you guessed it, you also have all that unburned oxygen. High O2 content in exhaust equals a lean reading!
There are also some other possibilities such as an internally leaking EGR system, (but this will typically set a separate code). A leak in the exhaust system before the O2 sensor will also cause incorrect readings. And always check for after- market modifications.
These can throw a wrench into the works! The only other possibilities (however unlikely), are wiring issues, computer concerns or a bad O2 sensor! There now that I've said it, on to rich codes.
The possible causes of rich codes are:
1. A leaking or faulty fuel injector
2. Fuel injector driver in computer shorted, or wiring short for injectors (likely a ground short)
3. Leaking or faulty fuel pressure regulator or restricted return line
4. Faulty evaporative emissions system - bleeding fuel vapors into engine (not commanded by computer)
5. On newer models a faulty fuel pump or fuel pump driver module
6. Faulty readings from other sensors such as a Mass Air Flow Sensor. You may actually be getting more air than the MAF tells the computer
7. Exhaust leaks before the sensor will cause erratic readings
8. After market components or performance chips
9. And yes, if I dare say it, possibly a computer, wiring issue or even a faulty O2 sensor!
The other codes we should address are those related to the sensors located after the catalytic converter. Though these may appear identical to the oxygen sensors pre-converter, they perform an entirely different task and are known as Monitors.
The only job of these sensors is to "monitor" the efficiency of the catalytic converters. The readings from these sensors should be much more stable and not fluctuating like the front O2 sensors.
The computer compares the readings from the oxygen sensors (pre cats) and the monitors (post cat) to determine if the catalytic converters are doing their job and "cleaning" the exhaust.
You never want to replace a monitor for a rich/lean concern as they have no bearing on these codes. As the converters begin to fail, you will see the monitors voltage readings follow the oxygen sensor readings.
Technically these are all "oxygen sensors" but it is important to distinguish the difference between pre-converter & post converter sensors, so I find it easiest to stick to calling the back ones monitors.
