Secondary Air Diagnostic

Secondary Air Diagnostic:
Through the secondary air system additional oxygen is introduced behind the exhaust valves to reduce harmful emissions. This occurs after starting the engine within a defined engine temperature range and for a predetermined time.

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

During the diagnosis of the secondary air system, additional air behind the exhaust valves creates a surplus of oxygen at the oxygen sensors causing a reduction of the voltage at the oxygen sensors ahead of the Three Way Catalytic Converter.

Oxygen Sensor Voltage Without Secondary Air:

Image Legend:
X = Oxygen sensor voltage
t = Time

Oxygen Sensor Voltage With Secondary Air:

Image Legend:
X = Oxygen sensor voltage
t = Time
A = Counter OK

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