3-way catalytic converter and lambda sensor

Pierburg | Motorservice

How do 3-way catalytic converters work? What do we need them for? What is a control sensor? What do we mean when we talk about a monitor probe? How do lambda sensors work? You can find the answers here.

Exhaust gas after-treatment

Raw emissions such as carbon monoxide, nitrogen oxides, sulphur dioxide and hydrocarbons are produced in the cylinders during combustion. The exhaust gas after-treatment converts these raw emissions into less harmful exhaust gases. Ideally, only nitrogen, carbon dioxide and water remain.
The classic exhaust gas after-treatment method is the “controlled 3-way catalytic converter”. Three chemical reactions of reduction and oxidation occur simultaneously here. The lambda is determined using the lambda sensor before the catalytic converter – also known as a control sensor. Taking this value as an input variable, the engine management system controls the addition of fuel to ensure efficient combustion.


How the lambda sensor works

The catalytic converter is monitored by the on-board diagnostics (OBD). If unburned fuel enters the exhaust gas, this can damage the catalytic converter. The catalytic converter also ages over time and gradually loses its ability to convert the pollutants. Therefore, a second lambda sensor after the catalytic converter monitors the condition of the catalytic converter. This second sensor is called a monitor probe.
Lambda sensors measure the oxygen content in the exhaust gas. If the signals from the control sensor and monitor probe are too similar, this indicates that almost no exhaust gas purification is taking place in the catalytic converter. The catalytic converter is defective and the OBD reports this. 

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How the 3-way catalytic converter and lambda sensor work

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