Secondary Air Injection System Monitoring: Description

The secondary air injection system consists of an electric pump that is controlled by the ECM via a relay. Air is supplied by the pump to two vacuum operated control valves, one per cylinder bank. From each of the control valves air is delivered to the exhaust ports of the center two cylinders of each cylinder bank. The vacuum signal is switched via an ECM controlled solenoid valve. A vacuum reservoir ensures that there is always sufficient depression to operate the control valves.

Diagnosis of the secondary air injection system can take place in two steps. There is a passive diagnostic which checks for a lean shift in the signals from the front oxygen sensors during secondary air injection operation and there is an active check, which only runs if the passive check fails to achieve sufficient test results in any drive cycle. The active test has two parts; firstly the secondary air injection pump will be run with the control valves shut. If the valves are leaking or stuck open, the feedback fuelling will shift lean and a fault will be detected. If the valve check is passed, then the valves will be opened and if sufficient secondary airflow exists, then the fuelling will be shifted lean. If the lean shift is less than the required threshold, then a fault is stored.

Additionally, a total absence of secondary injection airflow does not cause the vehicle to exceed the appropriate monitoring threshold. Therefore the system only requires a functional check for the presence of secondary air.