Oxygen sensor

The oxygen sensor (O2 sensor) measures the amount of oxygen in the exhaust and sends the signal to the engine computer. The front oxygen sensor is installed in the exhaust manifold or the front exhaust pipe in front of the catalytic converter. As you know, the catalytic converter is an important part of the vehicle emission control system.
A rear oxygen sensor is installed in the exhaust pipe behind the catalytic converter. Look at how the rear oxygen sensor looks inside the exhaust.
Cars with four cylinder engines 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 increasing or decreasing fuel. The rear oxygen sensor signal is used to monitor the performance of the catalytic converter.
The working principle of oxygen sensor
There are several types of oxygen sensors, but to keep them simple, we will only consider the voltage generated oxygen sensors. As the name suggests, the oxygen sensor produces a small voltage that is directly proportional to the difference in the amount of oxygen inside and outside the exhaust.
When the air fuel mixture entering the engine is thin (less fuel and more air), there is more oxygen in the exhaust, and the oxygen sensor will produce very small voltage (0.1-0.2V).
If the air fuel mixture becomes denser (more fuel and less air), the oxygen in the exhaust will decrease, so the oxygen sensor will generate more voltage (about 0.9V).
In order to work normally, the oxygen sensor must be heated to a certain temperature. A typical modern sensor has an internal electrical heating element supplied by the PCM.

Air fuel ratio adjustment
The front O2 sensor is responsible for keeping the air-fuel ratio of the blended gas entering the engine at the optimum level. For the 1 fuel, the air / fuel ratio is about 14.7:1 or 14.7 parts of the air.  When the current O2 sensor senses a high level of oxygen, the PCM assumes that the engine is running thin (not enough fuel), so PCM adds fuel. When the level of oxygen in the exhaust becomes low, PCM thinks the engine runs too much (too much fuel) and reduces fuel supply.
The process is continuous. The engine computer is constantly circulating between slightly thinner and slightly rich conditions to keep air / fuel ratios at optimum levels. This process is called closed loop operation. If you look at the voltage signal of the oxygen sensor in front (see the oscilloscope signal above), it will be circulating between 0.2 volts (Lean) and 0.9 volts (Rich).
When the car starts cold, the front oxygen sensor is not fully preheated, and the PCM does not use the front O2 sensor signal for fuel adjustment. This pattern is called the open loop. Only when the oxygen sensor is fully preheated, the fuel injection system enters the closed loop mode.
A new broadband air fuel ratio sensor is installed instead of a common oxygen sensor. The operating principle of air / fuel ratio sensors is different, but for the same purpose detecting whether the air / fuel mixture entering the engine is rich or dilute. The air fuel ratio sensor is more accurate and can measure a wider range of air fuel ratio. Please read the oxygen sensor, sensor identification and replacement on the next page.