Secondary Air System Monitor

Circuit Descriptions - Part 3

Secondary air system monitor
Automatic monitoring is carried out to ensure that the secondary air system is functioning correctly. For this purpose, operation of the secondary air injection and of he shut-off and air switch-over valves is monitored each time they are activated. The secondary air injection serves as an exhaust gas after treatment during the engine warm-up phase. For this purpose, fresh air is injected directly into the exhaust manifold to ensure the catalytic converter heats up at a faster rate.

Shortly after the engine is started, the secondary air pump is activated by the SLP relay. The time until it is switched on is dependent on the following fringe conditions:

- Engine temperature
- Load signal
- Engine speed

Monitoring principle
The oxygen sensor voltage is monitored in the engine control unit during activation of the secondary air pump. During problem-free operation of the secondary air system, the oxygen sensor voltage is primarily in the lean range.

At regular intervals (every 20 ms), the oxygen sensor voltage is registered within the control unit. Each measurement in which the oxygen sensor voltage is registered as being in the lean range is counted by an internal counter. If this count exceeds a predefined threshold, the system is recognized as being fully operational. If this threshold is not reached the engine control unit assumes there is a fault in the secondary air system. An entry is made in the fault code memory.