Oxygen Sensors - How they work & basic diagnosis

Messages
25,046
Location
ON, Canada eh?
I was trying to explain to a friend how Oxygen Sensors work and some basic troubleshooting he could do on his 2000ish Jeep and was looking for a link that he could print off and follow along with. I stumbled across this and thought I would post here for others that might find it helpful. (This isn't applicable to Air-Fuel Ratio sensors, which look like Oxygen Sensors but function a bit differently.)
Quote
Today's computerized engine control systems rely on inputs from a variety of sensors to regulate engine performance, emissions and other important functions. The sensors must provide accurate information otherwise driveability problems, increased fuel consumption and emission failures can result. One of the key sensors in this system is the oxygen sensor. It's often referred to as the "O2" sensor because O2 is the chemical formula for oxygen (oxygen atoms always travel in pairs, never alone). The first O2 sensor was introduced in 1976 on a Volvo 240. California vehicles got them next in 1980 when California's emission rules required lower emissions. Federal emission laws made O2 sensors virtually mandatory on all cars and light trucks built since 1981. And now that OBD-II regulations are here (1996 and newer vehicles), many vehicles are now equipped with multiple O2 sensors, some as many as four! The O2 sensor is mounted in the exhaust manifold to monitor how much unburned oxygen is in the exhaust as the exhaust exits the engine. Monitoring oxygen levels in the exhaust is a way of gauging the fuel mixture. It tells the computer if the fuel mixture is burning rich (less oxygen) or lean (more oxygen). A lot of factors can affect the relative richness or leanness of the fuel mixture, including air temperature, engine coolant temperature, barometric pressure, throttle position, air flow and engine load. There are other sensors to monitor these factors, too, but the O2 sensor is the master monitor for what's happening with the fuel mixture. Consequently, any problems with the O2 sensor can throw the whole system out of whack. Loops The computer uses the oxygen sensor's input to regulate the fuel mixture, which is referred to as the fuel "feedback control loop." The computer takes its cues from the O2 sensor and responds by changing the fuel mixture. This produces a corresponding change in the O2 sensor reading. This is referred to as "closed loop" operation because the computer is using the O2 sensor's input to regulate the fuel mixture. The result is a constant flip-flop back and forth from rich to lean which allows the catalytic converter to operate at peak efficiency while keeping the average overall fuel mixture in proper balance to minimize emissions. It's a complicated setup but it works. When no signal is received from the O2 sensor, as is the case when a cold engine is first started (or the 02 sensor fails), the computer orders a fixed (unchanging) rich fuel mixture. This is referred to as "open loop" operation because no input is used from the O2 sensor to regulate the fuel mixture. If the engine fails to go into closed loop when the O2 sensor reaches operating temperature, or drops out of closed loop because the O2 sensor's signal is lost, the engine will run too rich causing an increase in fuel consumption and emissions. A bad coolant sensor can also prevent the system from going into closed loop because the computer also considers engine coolant temperature when deciding whether or not to go into closed loop. How it Works The O2 sensor works like a miniature generator and produces its own voltage when it gets hot. Inside the vented cover on the end of the sensor that screws into the exhaust manifold is a zirconium ceramic bulb. The bulb is coated on the outside with a porous layer of platinum. Inside the bulb are two strips of platinum that serve as electrodes or contacts. The outside of the bulb is exposed to the hot gases in the exhaust while the inside of the bulb is vented internally through the sensor body to the outside atmosphere. Older style oxygen sensors actually have a small hole in the body shell so air can enter the sensor, but newer style O2 sensors "breathe" through their wire connectors and have no vent hole. It's hard to believe, but the tiny amount of space between the insulation and wire provides enough room for air to seep into the sensor (for this reason, grease should never be used on O2 sensor connectors because it can block the flow of air). Venting the sensor through the wires rather than with a hole in the body reduces the risk of dirt or water contamination that could foul the sensor from the inside and cause it to fail. The difference in oxygen levels between the exhaust and outside air within the sensor causes voltage to flow through the ceramic bulb. The greater the difference, the higher the voltage reading. An oxygen sensor will typically generate up to about 0.9 volts when the fuel mixture is rich and there is little unburned oxygen in the exhaust. When the mixture is lean, the sensor's output voltage will drop down to about 0.1 volts. When the air/fuel mixture is balanced or at the equilibrium point of about 14.7 to 1, the sensor will read around 0.45 volts. When the computer receives a rich signal (high voltage) from the O2 sensor, it leans the fuel mixture to reduce the sensor's reading. When the O2 sensor reading goes lean (low voltage), the computer reverses again making the fuel mixture go rich. This constant flip-flopping back and forth of the fuel mixture occurs with different speeds depending on the fuel system. The transition rate is slowest on engines with feedback carburetors, typically once per second at 2500 rpm. Engines with throttle body injection are somewhat faster (2 to 3 times per second at 2500 rpm), while engines with multiport injection are the fastest (5 to 7 times per second at 2500 rpm). The oxygen sensor must be hot (about 600 degrees or higher) before it will start to generate a voltage signal, so many oxygen sensors have a small heating element inside to help them reach operating temperature more quickly. The heating element can also prevent the sensor from cooling off too much during prolonged idle, which would cause the system to revert to open loop. Heated O2 sensors are used mostly in newer vehicles and typically have 3 or 4 wires. Older single wire O2 sensors do not have heaters. When replacing an O2 sensor, make sure it is the same type as the original (heated or unheated). A New Role for O2 Sensors with OBDII Starting with a few vehicles in 1994 and 1995, and all 1996 and newer vehicles, the number of oxygen sensors per engine has doubled. A second oxygen sensor is now used downstream of the catalytic converter to monitor the converter's operating efficiency. On V6 or V8 engines with dual exhausts, this means up to four O2 sensors (one for each cylinder bank and one after each converter) may be used. The OBDII system is designed to monitor the emissions performance of the engine. This includes keeping an eye on anything that might cause emissions to increase. The OBDII system compares the oxygen level readings of the O2 sensors before and after the converter to see if the converter is reducing the pollutants in the exhaust. If it sees little or no change in oxygen level readings, it means the converter is not working properly. This will cause the Malfunction Indicator Lamp (MIL) to come on. Sensor Diagnosis O2 sensors are amazingly rugged considering the operating environment they live in. But O2 sensors do wear out and eventually have to be replaced. The performance of the O2 sensor tends to diminish with age as contaminants accumulate on the sensor tip and gradually reduce its ability to produce voltage. This kind of deterioration can be caused by a variety of substances that find their way into the exhaust such as lead, silicone, sulfur, oil ash and even some fuel additives. The sensor can also be damaged by environmental factors such as water, splash from road salt, oil and dirt. As the sensor ages and becomes sluggish, the time it takes to react to changes in the air/fuel mixture slows down which causes emissions to go up. This happens because the flip-flopping of the fuel mixture is slowed down which reduces converter efficiency. The effect is more noticeable on engines with multiport fuel injection (MFI) than electronic carburetion or throttle body injection because the fuel ratio changes much more rapidly on MFI applications. If the sensor dies altogether, the result can be a fixed, rich fuel mixture. Default on most fuel injected applications is mid-range after three minutes. This causes a big jump in fuel consumption as well as emissions. And if the converter overheats because of the rich mixture, it may suffer damage. One EPA study found that 70% of the vehicles that failed an I/M 240 emissions test needed a new O2 sensor. The only way to know if the O2 sensor is doing its job is to inspect it regularly. That's why some vehicles (mostly imports) have a sensor maintenance reminder light. A good time to check the sensor is when the spark plugs are changed. You can read the O2 sensor's output with a scan tool or digital voltmeter, but the transitions are hard to see because the numbers jump around so much. Here's where a PC based scantool such as AutoTap really shines. You can use the graphing features to watch the transitions of the O2 sensors voltage. The software will display the sensor's voltage output as a wavy line that shows both it's amplitude (minimum and maximum voltage) as well as its frequency (transition rate from rich to lean). A good O2 sensor should produce an oscillating waveform at idle that makes voltage transitions from near minimum (0.1 v) to near maximum (0.9v). Making the fuel mixture artificially rich by feeding propane into the intake manifold should cause the sensor to respond almost immediately (within 100 milliseconds) and go to maximum (0.9v) output. Creating a lean mixture by opening a vacuum line should cause the sensor's output to drop to its minimum (0.1v) value. If the sensor doesn't flip-flop back and forth quickly enough, it may indicate a need for replacement. If the O2 sensor circuit opens, shorts or goes out of range, it may set a fault code and illuminate the Check Engine or Malfunction Indicator Lamp. If additional diagnosis reveals the sensor is defective, replacement is required. But many O2 sensors that are badly degraded continue to work well enough not to set a fault code-but not well enough to prevent an increase in emissions and fuel consumption. The absence of a fault code or warning lamp, therefore, does not mean the O2 sensor is functioning properly. Sensor Replacement Any O2 sensor that is defective obviously needs to be replaced. But there may also be benefits to replacing the O2 sensor periodically for preventive maintenance. Replacing an aging O2 sensor that has become sluggish can restore peak fuel efficiency, minimize exhaust emissions and prolong the life of the converter. Unheated 1 or 2 wire wire O2 sensors on 1976 through early 1990s vehicles can be replaced every 30,000 to 50,000 miles. Heated 3 and 4-wire O2 sensors on mid-1980s through mid-1990s applications can be changed every 60,000 miles. On OBDII equipped vehicles (1996 & up), a replacement interval of 100,000 miles is recommended. http://www.autotap.com/techlibrary/understanding_oxygen_sensors.asp
 
Last edited:
Messages
5,761
Location
New England
I worked on an interface circuit for a new type of EGO sensor for a major US OEM many years ago. The idea was that the sensor would produce more of a DC output in response to the oxygen level rather than slamming from high to low constantly...our interface circuit had to be very accurate to reflect this level appropriately to our internal circuitry that processed and digitized it for the micro running the engine. This was quite a task in the minimal space we could allot for the circuit and the customer's desired super-accurate specs were a constant bone of contention with our production group. I don't know if this type of sensor ever went anywhere, we put our interface circuit into a test chip and then as an option in a new generation of our interface part that processed outputs from many sensors...after which I left the company.
 

StevieC

Thread starter
Messages
25,046
Location
ON, Canada eh?
Originally Posted by Virtus_Probi
I worked on an interface circuit for a new type of EGO sensor for a major US OEM many years ago. The idea was that the sensor would produce more of a DC output in response to the oxygen level rather than slamming from high to low constantly...our interface circuit had to be very accurate to reflect this level appropriately to our internal circuitry that processed and digitized it for the micro running the engine. This was quite a task in the minimal space we could allot for the circuit and the customer's desired super-accurate specs were a constant bone of contention with our production group. I don't know if this type of sensor ever went anywhere, we put our interface circuit into a test chip and then as an option in a new generation of our interface part that processed outputs from many sensors...after which I left the company.
Cool.
 

StevieC

Thread starter
Messages
25,046
Location
ON, Canada eh?
Figured I might as well post this if this is a reference thread for 02's....
Quote
All You Ever Wanted To Know About Air Fuel Ratio Sensors It's not a part you think about often, but the air/fuel ratio sensor performs and important duty as a part of your car's emissions system. Here is the 411 on air/fuel ratio sensors. Understanding Air/Fuel Ratio Sensors An air/fuel ratio sensor is much like an oxygen sensor, or O2 sensor, and is slowly replacing them on many vehicles. Especially today's more sensitive, turbocharged, and efficient engines. Unlike the vulnerable O2 sensor, an air/fuel ratio sensor is a wide range sensor rather than narrow range, and it operates by conduction rather than generation and is therefore slightly more accurate. Like oxygen sensors, though, air/fuel ratio sensor will be positioned before and after the catalytic converter on OBDII-compliant engines (Sensor 1 is always on the exhaust manifold, Sensor 2 is always after the catalytic converter) and will be referred to as O2 sensors in most of the OBDII literature, despite being air/fuel ratio sensors. Air/Fuel Ratio Sensor vs. O2 Sensor Emission System at the Catalytic Converter The two differences in how an air/fuel ratio sensor works versus the older O2 sensor makes all the difference in how they function to help the electronic control module (ECM or ECU) tune the engine's performance. The older O2 sensors have a central core made of zirconium (or, in rare cases, titanium) which creates voltage when oxygen molecules pass through it. The new air/fuel ratio sensor uses a dual core and dedicated electronic circuitry to control current flow. The dual core acts as a conductor into which the air/fuel ratio sensor's circuitry sends current. The first of the dual cells, called the pump cell or diffusion chamber, receives air from the exhaust stream. Oxygen in that stream determines the conductivity of the cell, creating current differences from that which is being sent by the air/fuel ratio sensor's circuitry and being received back in the circuit. The second cell, the reference cell, houses outside air for reference in the calculations made by the ECM (engine control module). This allows a broader band of measurement as well as a more precise measure of what's happening in the exhaust stream in real-time. It also allows for faster adjustment. The ECM can change the pulsation-reduction to bring the mixture back to lean or widens injector pulsation to add fuel when the output is rich in oxygen. The air/fuel ratio itself can also have its current adjusted and even reversed in polarity in order to 'suck in' or expel excess oxygen from the Pump Cell to clear it for new measurements. All of these operations can be done several times per second, allowing for very precise control over the engine's output and emissions. The changes in voltage that the air/fuel ratio sensor measures are minuscule, on the order of milliamps, giving them high precision for measurement, but also making it difficult to test the air/fuel ratio sensor for diagnostics. Testing must be done using a scanner made for the purpose or a newer OBD scanner that has this functionality. In-line scanning via a meter is not possible with an air/fuel ratio due to the low changes being measured and the requirement that lines be cut and spliced to make the measurements (which could affect the input/output with added interference). It is necessary to know the normal operating voltage for the vehicle's air/fuel ratio sensor, usually in the 2.6.3.3 volt range, and 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 are lean, lower numbers are rich corresponding to the conduction difference in the Pump Cell. The Bottom Line on Air/Fuel Ratio Sensors Manufacturers are moving towards air/fuel ratio sensors because they give a faster, more control-able emissions control option, allowing a more continual optimal air:fuel ratio (usually held at 14.7:1 in a gasoline engine). Where an older O2 sensor may require more than a second to cause adjustments and calibration for air:fuel, an air/fuel ratio sensor allows it to be done in fractions of a second instead, greatly boosting economy. https://parts.olathetoyota.com/air-fuel-ratio-sensors
 

StevieC

Thread starter
Messages
25,046
Location
ON, Canada eh?
Originally Posted by Blueskies123
I have had many vehicles that go well over 200,000 miles and have never had to replace a sensor. I buy good gas and drive normally.
That has been my experience as well for the most part.
 
Messages
3,648
Location
Worst Case, Ontario
Originally Posted by StevieC
Originally Posted by Blueskies123
I have had many vehicles that go well over 200,000 miles and have never had to replace a sensor. I buy good gas and drive normally.
That has been my experience as well for the most part.
It might just be oil burners from bad design or poor break in then, or people replacing in vain to fix a P0420. I've also read about people changing them preemptively and gaining a few MPG. I thought a higher mileage car needing an o2 sensor was not unusual?
 

StevieC

Thread starter
Messages
25,046
Location
ON, Canada eh?
02 sensors can become less reliable in terms of values reported or lazy in terms of changing values frequently enough as they age, I've seen that many times but generally they are still within acceptable limits for the ECU. It depends how tight the regulations are in the ECU that determines if the light comes on or not. I know some early Chrysler you could hold the sensor just below the upper or lower threshold and it wouldn't set the light. So in a vehicle where more variance is accepted and they are replaced on a schedule versus when the light comes on can yield more MPG but it's not really that common and most MPG gains would be barely perceivable in any more modern vehicle using 02 sensors instead of AFR's.
 
Last edited:
Messages
9,966
Location
MA
Originally Posted by maxdustington
Originally Posted by StevieC
Originally Posted by Blueskies123
I have had many vehicles that go well over 200,000 miles and have never had to replace a sensor. I buy good gas and drive normally.
That has been my experience as well for the most part.
It might just be oil burners from bad design or poor break in then, or people replacing in vain to fix a P0420. I've also read about people changing them preemptively and gaining a few MPG. I thought a higher mileage car needing an o2 sensor was not unusual?
My car lasted a little bit over 200k. Changed the upstream O2 sensors at about 130k, gas mileage improved by about 1-2mpg. As the sensors were only $25 each and it only cost me $50 at an indy to replace both of them, I probably saved several times their cost in improved fuel mileage so even if they're not bad, new ones might easily pay for themselves. Oxygen sensor manufacturers themselves say to replace them every 50-60k although most manufacturers don't have a replacement interval mentioned.
 
Messages
10,001
Location
Waco, TX
Originally Posted by Wolf359
Changed the upstream O2 sensors at about 130k, gas mileage improved by about 1-2mpg. I probably saved several times their cost in improved fuel mileage so even if they're not bad, new ones might easily pay for themselves.
I don't get a CEL, but since my originals have 176,000 I am thinking of swapping them out for research purposes.
 

wdn

Messages
1,454
Location
NH
I have a scan tool that can graph a Autel MaxiCheck Pro and also a Autel MaxiLink ML619. They can also graph 2 readings on 2 charts or even overlay 2 readings on the same graph. The MaxiCheck Pro also has a O2 sensor test that shows both the max and min readings as text and some other reading I can't remember. I just got that one.
 

StevieC

Thread starter
Messages
25,046
Location
ON, Canada eh?
They are making some decent scanners. I have the MD805 at my house which does enough of what I need for friends/family and talks to all the modules, but my dad has a Snap On professional grade unit. He's a bit of a Snap-on nut. The guy still comes with the truck to his house from when he came to his shop. grin2
 
Last edited:
Messages
5,243
Location
Fort Worth, Texas
The "Sampling Rate" in the data stream can be slow, And/Or the scan tool itself can't "sample" the data stream fast enough. A 02 can seem lazy in scan data when it's not....That's why I prefer a Scope!
 

StevieC

Thread starter
Messages
25,046
Location
ON, Canada eh?
Originally Posted by clinebarger
The "Sampling Rate" in the data stream can be slow, And/Or the scan tool itself can't "sample" the data stream fast enough. A 02 can seem lazy in scan data when it's not....That's why I prefer a Scope!
My dad has an older scope and although it works it's not as good as the Pico which I would love to have but can't justify the cash they want for the thing and I would go with a Hantek instead but I've heard they aren't the greatest for some applications.
 
Messages
5,243
Location
Fort Worth, Texas
Originally Posted by StevieC
Originally Posted by clinebarger
The "Sampling Rate" in the data stream can be slow, And/Or the scan tool itself can't "sample" the data stream fast enough. A 02 can seem lazy in scan data when it's not....That's why I prefer a Scope!
My dad has an older scope and although it works it's not as good as the Pico which I would love to have but can't justify the cash they want for the thing and I would go with a Hantek instead but I've heard they aren't the greatest for some applications.
I have yet to run into something that "Requires" a Pico Scope, Sure the Sample Rate & Resolution is fantastic. On the other hand the.....Hooking up the box, Firing up the laptop, And setting the time bases & scales takes way to long! As a flat-rate technician......My Snap-On Vantage Pro is perfect.
 
Top