Audi A6 C5 4.2L V8 5V BAS Secondary Air Injection Maintenance

The secondary air system is designed to enable catalytic converter to heat up and reach its operating temperature more quickly after a cold start.
Principle of operation
Due to extra mixture enrichment during cold-start phase, an increased amount of unburnt hydrocarbons is carried in exhaust gas. The secondary air system improves the afterburning (oxidation) process in the catalytic converter, and in this way reduces toxic emissions. Heat generated by oxidation accelerates "light off" of catalytic converter and significantly improves exhaust gas quality during warm-up.
Function
After a cold start, engine control module -3- activates secondary air pump -1- via secondary air pump relay -2-, and air is fed to combination valves for secondary air system -4- and -8-.
At the same time, the system activates secondary air inlet valve - 5-, which supplies vacuum to combination valves -4- and -8-. The relevant combination valve opens a passage for the secondary air system to supply air to exhaust ports in cylinder head.

Source