Abstract: A system and method for detecting a failure occurring in an exhaust secondary air supply system of an internal combustion engine. A conduit is connected to the exhaust pipe to supply secondary air to the exhaust pipe to promote oxidization of the emissions in the exhaust gases. A valve is provided in the conduit for opening/closing it. The failure of the valve is detected by operating the air pump to supply the air in the conduit, while closing the conduit by the valve. The air/fuel ratio is detected by an oxygen sensor installed downstream of the conduit to be compared with a desired air/fuel ratio. If the detected air/fuel ratio is leaner than a desired air/fuel ratio, the air flows to the exhaust pipe so that the valve is considered to be stuck to open. Alternatively, a pump current is detected and is compared with a reference value indicative of the current at no air flow. If the detected current is less than the reference value, the valve is considered to be in failure.