Immediately after starting a cold spark-ignition (petrol) engine, until Lambda control starts to take effect, undesirable quantities of harmful HC and CO are emitted.  The use of a secondary-air system can significantly reduce the level of such pollutants in the cold starting phase.

The secondary air system supplies ambient air to the exhaust when the engine first starts up, until the engine is warm enough for the oxygen sensor to take control over the combustion mixture. PIERBURG describes the basic function of the secondary air system and how they work.

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