Updated: August 16, 2013
Heated oxygen sensor. Click for larger photo
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An oxygen sensor (O2 sensor) measures the amount of oxygen in the exhaust gases, sending the signal to the engine computer. A front oxygen sensor is installed in the exhaust manifold or in the front exhaust pipe before the catalytic converter. As you know, a catalytic converter is a major part of the vehicle's emission control system.
A rear oxygen sensor is mounted in the exhaust pipe after the catalytic converter. See the photo showing how the rear oxygen sensor looks inside the exhaust.
Cars with a 4-cylinder engine have at least two oxygen sensors; V6 and V8 vehicles have at least four O2 sensors.
The engine computer (Powertrain Control Module, or PCM) uses the signal from the front oxygen sensor to adjust the air/fuel ratio by adding or subtracting fuel. The rear oxygen sensor signal is used to monitor the performance of the catalytic converter.
A rear oxygen sensor is mounted in the exhaust pipe after the catalytic converter. See the photo showing how the rear oxygen sensor looks inside the exhaust.
Cars with a 4-cylinder engine have at least two oxygen sensors; V6 and V8 vehicles have at least four O2 sensors.
The engine computer (Powertrain Control Module, or PCM) uses the signal from the front oxygen sensor to adjust the air/fuel ratio by adding or subtracting fuel. The rear oxygen sensor signal is used to monitor the performance of the catalytic converter.
Front (upstream) oxygen sensor
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The engine computer adjusts air/fuel ratio
based on the feedback from the front O2 sensor |
Front oxygen sensor voltage signal
on a scope display. Click for larger photo |
How Oxygen Sensor works
There are a few types of oxygen sensors, but to keep it simple, we will only consider voltage-generating oxygen sensors. As the name implies, a voltage-generating oxygen sensor generates a small voltage proportional to the difference in the amount of oxygen inside and outside of the exhaust.
When the air-fuel mixture entering the engine is lean (less fuel and more air), there is more oxygen in the exhaust and the oxygen sensor will generate a very small voltage (0.1 - 0.2V).
If the air-fuel mixture becomes rich (more fuel and less air), there is less oxygen in the exhaust, so the oxygen sensor will generate more voltage (around 0.9V).
To work properly, an oxygen sensor must be heated to a certain temperature. A typical modern sensor has an internal electric heating element that is powered by the PCM.
When the air-fuel mixture entering the engine is lean (less fuel and more air), there is more oxygen in the exhaust and the oxygen sensor will generate a very small voltage (0.1 - 0.2V).
If the air-fuel mixture becomes rich (more fuel and less air), there is less oxygen in the exhaust, so the oxygen sensor will generate more voltage (around 0.9V).
To work properly, an oxygen sensor must be heated to a certain temperature. A typical modern sensor has an internal electric heating element that is powered by the PCM.
Air/fuel Ratio Adjustment
A front O2 sensor is responsible for keeping the air/fuel ratio of the mixture entering the engine at the optimal level, which is approximately 14.7:1 or 14.7 parts of air to 1 part of fuel. When the front O2 sensor senses high level of oxygen, the PCM assumes that the engine is running lean (not enough fuel), so the PCM adds fuel. When the level of oxygen in the exhaust becomes low, the PCM assumes that the engine is running rich (too much fuel) and reduces fuel supply.
This process is continuous. The engine computer constantly cycles between slightly lean and slightly rich conditions to keep the air/fuel ratio at the optimum level. This process is called closed loop operation. If you look at the front oxygen sensor voltage signal (see the scope signal above), it will be cycling somewhere between 0.2 Volts (Lean) and 0.9 Volts (Rich).
When the car is started cold, the front oxygen sensor is not fully warmed up, and the PCM does not use the front O2 sensor signal for fuel trim adjustment. This mode is called an open loop. Only when the oxygen sensor is fully warmed up, the fuel injection system goes into the closed loop mode.
Newer cars have a broadband air/fuel ratio sensor installed instead of a regular oxygen sensor. An air/fuel ratio sensor works differently, but serves the same purpose - to detect whether the air/fuel mixture entering the engine is rich or lean. An air-fuel ratio sensor is more precise and can measure wider range of air/fuel ratio. Read about rear oxygen sensor, sensor identification and replacement on the next page.
This process is continuous. The engine computer constantly cycles between slightly lean and slightly rich conditions to keep the air/fuel ratio at the optimum level. This process is called closed loop operation. If you look at the front oxygen sensor voltage signal (see the scope signal above), it will be cycling somewhere between 0.2 Volts (Lean) and 0.9 Volts (Rich).
When the car is started cold, the front oxygen sensor is not fully warmed up, and the PCM does not use the front O2 sensor signal for fuel trim adjustment. This mode is called an open loop. Only when the oxygen sensor is fully warmed up, the fuel injection system goes into the closed loop mode.
Newer cars have a broadband air/fuel ratio sensor installed instead of a regular oxygen sensor. An air/fuel ratio sensor works differently, but serves the same purpose - to detect whether the air/fuel mixture entering the engine is rich or lean. An air-fuel ratio sensor is more precise and can measure wider range of air/fuel ratio. Read about rear oxygen sensor, sensor identification and replacement on the next page.
Rear Oxygen Sensor
Rear (downstream) oxygen sensor diagram
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A rear or downstream oxygen sensor is installed in the exhaust after the catalytic converter. It measures the amount of oxygen in the exhaust gases coming out of the catalytic converter. The signal from the rear oxygen sensor is used to monitor the efficiency of the catalytic converter.
The engine computer or PCM constantly compares the signals from the front and the rear oxygen sensors (see the diagram). Based on the two signals, the PCM knows how well the catalytic converter does its job. If the catalytic converter fails, the PCM turns on the "Check Engine" light to let you know.
The rear oxygen sensor can be checked with a scan tool or a lab scope.
The engine computer or PCM constantly compares the signals from the front and the rear oxygen sensors (see the diagram). Based on the two signals, the PCM knows how well the catalytic converter does its job. If the catalytic converter fails, the PCM turns on the "Check Engine" light to let you know.
The rear oxygen sensor can be checked with a scan tool or a lab scope.
Oxygen Sensor Identification
Typically, the engine bank that
contains cylinder 1 is called Bank 1 |
The front oxygen sensor before the catalytic converter is usually called 'upstream' or Sensor 1.
The rear oxygen sensor installed after the catalytic converter is called 'downstream' or Sensor 2.
A typical in-line 4-cylinder engine has only one bank (Bank 1). Therefore, in an in-line 4-cylinder engine, the term "Bank 1, Sensor 1" simply refers to the front oxygen sensor. "Bank 1, Sensor 2" is the rear oxygen sensor.
A V6 or V8 engine has two banks (or two parts of that "V"). Usually, the bank that contains the cylinder number 1 is called "Bank 1."
Different car manufacturers define Bank 1 and Bank 2 differently. To know which is Bank 1 and Bank 2 in your car, you can look it up in a repair manual or you can google it, mentioning the year, make, model and engine size of your car. For example, according to Toyota bulletin TSB-0398-09, in a V6 Camry, Highlander, Avalon, Sienna and Solara, bank 1 is at the back, bank 2 is in the front. Similarly, in a V6 2003-2008 Mazda 6 or V6 Mazda Tribute, bank 1 is at the back, bank 2 is at the front. In a 2003 Nissan Maxima, bank 1 is at the back, bank 2 is in the front.
The rear oxygen sensor installed after the catalytic converter is called 'downstream' or Sensor 2.
A typical in-line 4-cylinder engine has only one bank (Bank 1). Therefore, in an in-line 4-cylinder engine, the term "Bank 1, Sensor 1" simply refers to the front oxygen sensor. "Bank 1, Sensor 2" is the rear oxygen sensor.
A V6 or V8 engine has two banks (or two parts of that "V"). Usually, the bank that contains the cylinder number 1 is called "Bank 1."
Different car manufacturers define Bank 1 and Bank 2 differently. To know which is Bank 1 and Bank 2 in your car, you can look it up in a repair manual or you can google it, mentioning the year, make, model and engine size of your car. For example, according to Toyota bulletin TSB-0398-09, in a V6 Camry, Highlander, Avalon, Sienna and Solara, bank 1 is at the back, bank 2 is in the front. Similarly, in a V6 2003-2008 Mazda 6 or V6 Mazda Tribute, bank 1 is at the back, bank 2 is at the front. In a 2003 Nissan Maxima, bank 1 is at the back, bank 2 is in the front.
Replacing an Oxygen Sensor
Oxygen sensor problems are common. A faulty oxygen sensor may cause poor gas mileage, emission test failure and various driveability problems (hesitation, poor acceleration, surging, etc.). If an oxygen sensor is bad, it needs to be replaced.
Replacing the oxygen sensor
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In most cars, replacing an oxygen sensor is a fairly simple procedure. In a repair shop, oxygen sensor replacement costs from $50 to $250 (labor only). If you want to replace the oxygen sensor yourself, with some skills and a repair manual, it's not so difficult, but you might need a specialoxygen sensor socket (in the photo).
Sometimes it might be hard to get the old sensor out, as the oxygen sensors are often rusted in place pretty bad. Another thing to be aware of, some cars are known to have troubles with aftermarket oxygen sensors. For example, there were a number of reports about non-original oxygen sensor causing problems in some Chrysler engines. If you are not sure, it's always best to use the original oxygen sensor that you can buy from your dealer.
Question: My 02 Buick Regal Check Engine light is on. I took it to an auto store they scanned with a scanner, it read oxygen sensor. I replaced the top and bottom and the light still on; it keep reading the oxygen sensor still. What could be the problem, please help.
A: The code for the oxygen sensor does not tell directly that the sensor is bad. The trouble code only tells what signal is out of range, it doesn't tell why. It would also help if you'd actually mention what the code was, for example, P0131. What I'd recommend, is to take your vehicle to a mechanic that has a more advanced scanner, the one that not only reads the code, but can also display live parameters when the engine is running. Looking at the live oxygen sensor signal and other parameters, such as mass airflow, long-term fuel trim and some others, it would be easy to see what is the problem. Many other issues can trigger a code for an oxygen sensor. For example, a clogged catalytic converter, dirty mass airflow sensor, vacuum leaks, etc. You can take your car to a GM dealer, at least just for diagnostic. I'm sure it will be more efficient way than just throwing new parts without diagnosing the actual problem.
Sometimes it might be hard to get the old sensor out, as the oxygen sensors are often rusted in place pretty bad. Another thing to be aware of, some cars are known to have troubles with aftermarket oxygen sensors. For example, there were a number of reports about non-original oxygen sensor causing problems in some Chrysler engines. If you are not sure, it's always best to use the original oxygen sensor that you can buy from your dealer.
Question: My 02 Buick Regal Check Engine light is on. I took it to an auto store they scanned with a scanner, it read oxygen sensor. I replaced the top and bottom and the light still on; it keep reading the oxygen sensor still. What could be the problem, please help.
A: The code for the oxygen sensor does not tell directly that the sensor is bad. The trouble code only tells what signal is out of range, it doesn't tell why. It would also help if you'd actually mention what the code was, for example, P0131. What I'd recommend, is to take your vehicle to a mechanic that has a more advanced scanner, the one that not only reads the code, but can also display live parameters when the engine is running. Looking at the live oxygen sensor signal and other parameters, such as mass airflow, long-term fuel trim and some others, it would be easy to see what is the problem. Many other issues can trigger a code for an oxygen sensor. For example, a clogged catalytic converter, dirty mass airflow sensor, vacuum leaks, etc. You can take your car to a GM dealer, at least just for diagnostic. I'm sure it will be more efficient way than just throwing new parts without diagnosing the actual problem.
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