check engine AUDI A4 1995 B5 / 1.G AHA ATQ Engines Secondary Air System Workshop Manual

26-13

Flow test
During the flow test, the oxygen sensor control is
switched off (engine runs rich at that time). At the
same time the secondary air injection pump is
switched on, injecting fresh air into the exhaust
manifold which increases the oxygen content in
the exhaust gas. The oxygen sensor now
measures a lean mixture. To pass the diagnostic
test, the oxygen sensor must measure a lean
mixture for a certain time. Passing the test
means that the secondary air injection system
delivers enough air and the secondary air
injection system is OK.

Leak test

Flow test must check out OK
Engine at operating temperature
The secondary air injection pump is switched on
during idle. Since the combination valves are not
triggered by the secondary air injection solenoid
valve, they remain closed. When the secondary
air injection pump is switched on, the oxygen
sensor system is monitored by the ECM. If the
system does not leak, fresh air cannot enter the
exhaust manifold. If the system is leaking, the
oxygen sensor signal will respond to the leaner
mixture (caused by the secondary air getting into
the exhaust manifold) and cause a noticeable
Pa
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