2003 3BMXV02.5M56
General Description - At cold start, secondary air pump and valve are switched on for their normal operating function. Secondary air delivered into exhaust gas is measured by a separate (small) mass airflow sensor, which is mounted into air tube between secondary air pump and secondary air filter. Anytime mass airflow sensor indicates fixed minimum or maximum air mass values in dependence of engine speed during a defined period, a fault will be detected. Fixed minimum air mass values correspond to minimum amount of secondary air (low flow limit check) affected by a defective pump. Fixed maximum air mass value corresponds to maximum amount of secondary air (high flow limit check) affected by a leakage between pump and manifold. If signal of secondary mass airflow sensor is stuck, a fault will be detected. When measuring a certain oscillation of secondary mass airflow, it can be detected whether secondary air valve is jammed open or blocked. See Fig 1.