Oxygen sensor

An oxygen sensor (O2 sensor) measures the amount of the oxygen in the exhaust gases, sending the voltage signal to the engine computer (ECM). The ECM uses the signal from the front and the rear oxygen sensors to control the engine operation and to monitor the performance of the catalytic converter, which is a major part of the vehicle's emission control system. A typical modern car has two or four oxygen sensors in the exhaust: one in front of the catalytic converter(s) and one after.

How an oxygen sensor works

Heated oxygen sensor. Click for larger photo

There are few types of oxygen sensors, but here we will consider a commonly used, the voltage-generating type. As the name implies, a voltage-generating oxygen sensor generates 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). This is because the difference between the amount of oxygen inside the exhaust is small compared to the outside. 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). By measuring the oxygen sensor voltage, the engine computer (ECM) knows if the air/fuel mixture is lean or rich. To work properly, an oxygen sensor must be heated to a certain temperature. In the 80-s, an oxygen sensor was heated by the hot exhaust gases. Newer cars use heated oxygen sensors, with an electric heating element inside.

Front oxygen sensor.

Front (upstream) oxygen sensor

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

A front oxygen sensor is the key component of an electronic fuel injection system. The front O2 sensor is responsible for keeping the air-fuel ratio of the mixture entering the engine at an optimal level, which is approximately 14.7:1.
A front oxygen sensor is installed in the exhaust manifold or in the exhaust down pipe, close to the engine, right before the catalytic converter. It monitors the amount of the oxygen in the exhaust gases and provides the feedback signal to the engine computer whether the air-fuel mixture is lean or rich (see the diagram).
When the front O2 sensor senses high level of oxygen, the ECM assumes that the engine is running lean (not enough fuel), so the ECM adds a little more fuel.
When the level of oxygen in the exhaust becomes low, the ECM assumes that the engine is running rich (too much fuel) and reduces fuel supply accordingly.
This process is continuous. The engine computer constantly cycles between slightly lean and slightly rich conditions to keep the air/fuel ratio at the optimal level. This process is called closed loop operation.
When the car is started cold, the front oxygen sensor is not fully warmed up, and the ECM does not use the front O2 sensor signal for fuel trim adjustment. This mode is called open loop. Only when the oxygen sensor is fully warmed up, the fuel injection system goes into closed loop mode.
If you look at the front oxygen sensor voltage signal (in the photo), it will be cycling somewhere between 0.2 Volts (Lean) and 0.9 Volts (Rich).
Some 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.

Rear oxygen sensor

Rear (downstream) oxygen sensor diagram

Rear oxygen sensor voltage signal
on a scope display. Spikes show rich
mixture when the engine is revved.

The rear oxygen sensor is installed in the exhaust after the catalytic converter. The rear oxygen sensor is used to monitor the efficiency of the catalytic converter.
As mentioned before, the engine computer (ECM) constantly cycles between slightly lean and slightly rich air/fuel mixture, so the level of oxygen before the catalytic converter constantly changes. You can see it on the graph in the previous paragraph.
If the catalytic converter works properly, the level of oxygen after the catalytic converter should be close to constant.
The ECM constantly compares the signals from the front and the rear oxygen sensors. If the signal from the rear oxygen sensor shows that the catalytic converter can no longer do its job properly, the ECM turns on the "Check Engine" light and sets the corresponding trouble code in its memory.
The rear oxygen sensor can be checked with a scan tool or a lab scope. You can see the rear oxygen sensor voltage signal on the graph. Spikes indicate the rich mixture when the engine was revved.

Bank 1, Bank 2

Typically, the engine bank that
contains cylinder 1 is called Bank 1

A typical in-line 4-cylinder engine has only one bank of four cylinders (Bank 1) and two oxygen sensors, one in front of the catalytic converter (in a technical literature it's called "Front O2 sensor" or "Sensor 1") and one after it ("Rear O2 sensor" or "Sensor 2"). 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"), each with three or four cylinders respectively. Usually, in a V6 or V8 engine, the bank that contains the cylinder number 1 is called "Bank 1" and the opposite bank is called "Bank 2". A typical V6 or V8 engine has two catalytic converters and four oxygen sensors: "Sensor 1" or "Front O2 sensor" refers to a sensor closer to the engine, before the catalytic converter and "Sensor 2" or "Rear O2 Sensor" is the one after. For example, the Bank 1 Sensor 2 is the rear oxygen sensor of the bank containing the cylinder number 1.

Oxygen sensor problems

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 the oxygen sensor has a problem, the "check engine" light on the instrument panel will come on. Here are few examples of engine trouble codes that could be cause by a problem with an oxygen sensor:
P0130 - O2 Sensor Circuit (Bank 1 Sensor 1)
P0131 - O2 Sensor Circuit Low Voltage (Bank 1 Sensor 1)
P0132 - O2 Sensor Circuit High Voltage (Bank 1 Sensor 1)
P0133 - O2 Circuit Slow Response (Bank 1, Sensor 1)
P0134 - O2 Sensor No Activity Detected (Bank 1 Sensor 1)
P0135 - O2 Sensor Heater Circuit (Bank 1 Sensor 1)
These are examples of the codes related to the front oxygen sensor of the Bank 1. There are separate codes for the rear oxygen sensor and the sensors of the Bank 2.

Oxygen sensor replacement

Oxygen Sensor Socket

Replacing the oxygen sensor

For most cars, the oxygen sensor replacement is a fairly simple and usually not very expensive procedure in a repair shop. 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 special oxygen sensor socket (in the photo). Sometimes it might be difficult to get the old sensor out, as the oxygen sensors are often rusted in place pretty bad. Another thing to consider, 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.
If you are looking for Do-It-Yourself car repair instructions, there is a website where for a small fee you can get an on-line access to a repair information for your car. read more »

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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