Circuit Description

Secondary Air Injection (AIR) system reduces hydrocarbons, carbon monoxide and oxides of nitrogen exhaust emissions. When engine is started, ECM commands AIR pump relay and AIR solenoid valve on, injecting clean air into engine exhaust ports. ECM switches AIR system off when oxygen sensors become active. Airflow is controlled by vacuum operated cutoff valve. Vacuum to valve is controlled by AIR solenoid valve. A check valve close to engine prevents hot exhaust gasses from backing up into AIR system.

ECM uses short term fuel trim to diagnose AIR system. When AIR system is enabled, ECM monitors short term fuel trim. If short term fuel trim goes below a threshold, ECM interprets this as an indication that AIR system is operational. When AIR pump is disabled, short term fuel trim should return to normal.

Conditions required to set DTC are:

• Short term fuel trim remains below a predetermined value for at least 20 seconds.