Abstract: An estimation unit estimates a fuel pressure on a discharge side of a high-pressure pump based on an amount of fuel injection with a fuel injection valve from a time point at which a detected fuel pressure changes from a higher status than a first pressure to a lower status, assuming that the fuel has not been discharged with the high-pressure pump. The determination unit determines that a relief valve is in an open status based on a dropping mode of the estimated fuel pressure and a dropping mode of the detected fuel pressure.

Abstract: An estimation unit estimates a fuel pressure on a discharge side of a high-pressure pump based on an amount of fuel injection with a fuel injection valve from a time point at which a detected fuel pressure changes from a higher status than a first pressure to a lower status, assuming that the fuel has not been discharged with the high-pressure pump. The determination unit determines that a relief valve is in an open status based on a dropping mode of the estimated fuel pressure and a dropping mode of the detected fuel pressure.

Abstract: A main feedback control based on an output of an upstream sensor, and a sub-feedback control based on an output of a downstream sensor are executed. A specified fuel disturbance is inputted and an output of the downstream sensor is measured to derive a characteristic value. Based on the characteristic value, it is determined whether a purifying capacity of the exhaust gas purifying system including the catalyst, the upstream sensor, and the downstream sensor is deteriorated.

Abstract: When an output of a downstream sensor provided downstream of a catalyst for exhaust gas purification becomes a leaner value than a leanness determination value, an air-fuel ratio control device supplies a rich component to the catalyst by performing rich input processing, in which increase correction for increasing fuel injection quantity stepwise is performed and then increase correction quantity of the fuel injection quantity is decreased gradually. When the increase correction quantity of the fuel injection quantity defined by the rich input processing becomes zero or when oxygen occlusion quantity of the catalyst becomes zero, the control device supplies a lean component to the catalyst by performing lean input processing, in which decrease correction for decreasing the fuel injection quantity stepwise is performed and then decrease correction quantity of the fuel injection quantity is decreased gradually.

Abstract: A main feedback control based on an output of an upstream sensor, and a sub-feedback control based on an output of a downstream sensor are executed. A specified fuel disturbance is inputted and an output of the downstream sensor is measured to derive a characteristic value. Based on the characteristic value, it is determined whether a purifying capacity of the exhaust gas purifying system including the catalyst, the upstream sensor, and the downstream sensor is deteriorated.