Secondary air supply system and abnormality diagnosis method of secondary air supply system

摘要

In a secondary air supply system, each of two branch portions of a secondary air supply pipe that is divided from a joint portion is provided with an air control valve, and the joint portion is provided with a pressure sensor and an auxiliary air control valve. The air control valves are opened at different timings such that pressure fluctuations in the joint portion upon opening of the firstly and the secondly opened air control valves are measured. An abnormality diagnosis is performed with respect to those two air control valves based on combinations of the aforementioned two pressure fluctuations.