Oxygen Sensor: Description and Operation

Oxygen Sensors

The oxygen sensors play a role in the mixture control and the exhaust analysis. It sends a signal to the Engine Control Module (ECM) indicating if the engine is running either rich or lean. The ECM uses this signal in combination with other engine management component signals to adjust mixture. This occurs only during closed loop operation.

There are two types of oxygen sensors:

• 4 wire sensors

• 6 wire sensors (broadband)

Pre-catalyst oxygen sensors monitor combustion efficiency, and are located in the exhaust before the catalytic converter.

Post-catalyst oxygen sensors monitor catalytic converter efficiency, and are located in the exhaust at the outlet of the catalytic converter.

The oxygen sensors measure the oxygen content of the exhaust to obtain an instantaneous indication of engine operation. The sensors must be at operating temperature to be accurate. Because the oxygen sensor needs to reach operating temperature quickly, a heating element is integrated into the sensor. This enable the temperature to be maintained despite changing engine operating conditions.

Closed Loop Operation

When the engine is running rich, the oxygen content in the exhaust will be low and reduction can occur. When it is running lean, the oxygen content will be high and oxidation can occur. This is called "closed loop" operation. because the cycle repeats in an continual "loop".

The pre-catalyst oxygen sensor is used to monitor the oxygen level in the exhaust during closed loop operation. This data is then sent to the ECM which adjusts the duration of the injector to provide the correct levels of oxygen for reduction and oxidation. The status of the regulation, called "mixture control", can be seen in Measured Valve Blocks 32 and 33.

There is also a mode of operation called "open loop". This occurs during the period of time before the oxygen sensor reaches operating temperature, generally after engine start, and lasts for only a brief period of time. (Most modern vehicles can begin closed loop operation in less than a minute, even after a cold start.) During open loop operation, mixture control is managed by a map inside the ECM and the mixture does not alternate rich/lean.

4 Wire Oxygen Sensors

The key element is a ceramic body which is coated on both sides. This coating functions as an electrode, where the electrode layer on one side comes into contact with external air and the other with the exhaust. A voltage difference is established between the electrodes by differing concentrations of oxygen in the external air and the exhaust. This voltage is evaluated to determine the lambda valve in the ECM.

Oxygen sensor voltage goes up due to a rich mixture, and down due to a lean mixture.

4 wire oxygen sensors generate a voltage signal which indicates exhaust oxygen content. If the oxygen is high (lean mixture) the voltage will be low. If the oxygen content is low (rich mixture) the voltage will be high.

On earlier models, the 4 wire oxygen sensors were used in front of the catalytic converter. Currently, the 4 wire oxygen sensors are only used behind the catalytic converter.

6 Wire Oxygen Sensors

The 6 wire (broadband) sensor actually contains two different components:

• The first component works like a traditional 4 wire oxygen sensor where voltage goes up when there is a lack of oxygen (rich) and voltage goes down when there is an excess of oxygen (lean).

• The second component is called a pump, but it acts more like an electronic gate. Think of it like a piece of "smart glass". When current is applied to a piece of smart glass, the glass changes from transparent to completely opaque, thus controlling the amount of light passing through it. In the same way, the pump in a 6 wire sensor allows more or less oxygen to pass through it when a positive or negative current is applies to it.

If the oxygen sensor reads a lean mixture (low voltage), the ECM will apply a positive current to the pump, less oxygen is allowed to pass through it and into the diffusing chamber, thus increasing the voltage of the oxygen sensor. If the oxygen sensor reads a rich mixture the control module will reverse the polarity to the pump and allow more oxygen to pass through it and the voltage of the oxygen sensor will drop. The ECM monitors the amount of current required to operate the pump and uses this reading to control the injector duration.