1. Field of the Invention
The present invention relates to an apparatus and method for diagnosis of a vehicular system such as an intake system and an exhaust system of an internal combustion engine.
2. Description of Related Art
Conventionally, a motor vehicle has a plurality of system such as an engine intake system, an engine exhaust system, a brake system and a navigation system. Since those systems are becoming complex, it is necessary to equip a self-diagnosis function on those systems in order to prevent an abnormal operation of those systems. However, it is not easy to determine a failed component when the system functions normally, since it is also required to keep normal function of the system. In addition, since each system has a plurality of components of which functions are influenced each other, reliable determination of the failed component is hardly achieved while continuing the normal function of the system.
For example, the exhaust system has a catalyst and a gas sensor. However, a deterioration level and an activation level of the catalyst influence an output of the gas sensor. In order to reduce the influence, JP-A-9-33478 discloses a diagnosis apparatus that sets a threshold value for determining a deterioration of the gas sensor in accordance with the deterioration level and the activation level of the catalyst. However, since the deterioration level of the catalyst also has an error, reliable determination is hardly achieved.