Air Injection: Description and Operation

Diagnosis of Secondary Air System

Through the secondary air system, additional oxygen is introduced behind the exhaust valves in order to reduce harmful emissions. This occurs after the engine is started, within a defined engine temperature range and for a predetermined time.

While secondary air injection is active, passive diagnosis of the system is performed.

During passive diagnosis of the secondary air system, the additional air behind the exhaust valves leads to a surplus of oxygen in the oxygen sensors and therefore to a voltage reduction in oxygen sensors ahead of the TWC.







The diagnosis checks in a predetermined rpm/load and temperature range whether the sensor voltage is too low due to secondary air or too high because of a lack of secondary air.







If passive diagnosis is interrupted, e.g. due to excessive rpm, then active diagnosis is performed at idle speed.
During active diagnosis, the DME switches the secondary air pump on when the engine has reached operating temperature. Here, the deviation of the oxygen sensor is observed and, in this way, the secondary air system assessed.