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Secondary air system - overview > < Servicing secondary air system

A4 Mk2

Principle and function
Principle and function
Principle
Because of the over-enrichment of the mixture in the cold start phase, the proportion of unburned hydrocarbons in the exhaust gas is higher. The secondary air system improves the afterburning (oxidation) process in the catalytic converter, and thus reduces toxic emissions. The heat generated by oxidisation accelerates the “light off” of the catalytic converter and significantly improves exhaust gas quality during warm-up.
   
Function
t  During the warm-up phase, the engine control unit -3- activates the secondary air pump -1- via the secondary air pump relay -2-. Air is directed to the combination valves for secondary air system -4- and -8-.
t  At the same time, the secondary air inlet valve -5- is actuated, thus allowing vacuum to reach the combination valves for secondary air system -4- and -8-. In this way, the combination valve opens a passage for the secondary air system to supply air to the exhaust ports in the cylinder head.

   

Secondary air system - overview > < Servicing secondary air system