Abstract: A secondary air supply system includes a supply passage, a valve, and a detecting unit. Secondary air is supplied to the upstream of a purification apparatus in an exhaust passage of an engine through the supply passage. The valve communicates and blocks the supply passage to be in open and close conditions. The detecting unit detects a detection object of secondary air in the upstream of the valve in the supply passage. A diagnosis apparatus for the secondary air supply system includes a diagnosis unit that obtains a detection result of the detecting unit in each of the open and close conditions of the valve for evaluating an abnormity of the secondary air supply system in accordance with the detection result. The diagnosis apparatus includes a reliability maintaining unit that determines the diagnosis operation of the diagnosis unit to be valid when the detecting operations in the open and close conditions are continuously performed.

Abstract: A secondary air supply system includes a supply passage, a valve, and a detecting unit. Secondary air is supplied to the upstream of a purification apparatus in an exhaust passage of an engine through the supply passage. The valve communicates and blocks the supply passage to be in open and close conditions. The detecting unit detects a detection object of secondary air in the upstream of the valve in the supply passage. A diagnosis apparatus for the secondary air supply system includes a diagnosis unit that obtains a detection result of the detecting unit in each of the open and close conditions of the valve for evaluating an abnormity of the secondary air supply system in accordance with the detection result. The diagnosis apparatus includes a reliability maintaining unit that determines the diagnosis operation of the diagnosis unit to be valid when the detecting operations in the open and close conditions are continuously performed.

Abstract: A secondary air supply system includes a supply passage, a valve, and a detecting unit. Secondary air is supplied to the upstream of a purification apparatus in an exhaust passage of an engine through the supply passage. The valve communicates and blocks the supply passage to be in open and close conditions. The detecting unit detects a detection object of secondary air in the upstream of the valve in the supply passage. A diagnosis apparatus for the secondary air supply system includes a diagnosis unit that obtains a detection result of the detecting unit in each of the open and close conditions of the valve for evaluating an abnormity of the secondary air supply system in accordance with the detection result. The diagnosis apparatus includes a reliability maintaining unit that determines the diagnosis operation of the diagnosis unit to be valid when the detecting operations in the open and close conditions are continuously performed.

Abstract: A secondary air supply system includes a supply passage, a valve, and a detecting unit. Secondary air is supplied to the upstream of a purification apparatus in an exhaust passage of an engine through the supply passage. The valve communicates and blocks the supply passage to be in open and close conditions. The detecting unit detects a detection object of secondary air in the upstream of the valve in the supply passage. A diagnosis apparatus for the secondary air supply system includes a diagnosis unit that obtains a detection result of the detecting unit in each of the open and close conditions of the valve for evaluating an abnormity of the secondary air supply system in accordance with the detection result. The diagnosis apparatus includes a reliability maintaining unit that determines the diagnosis operation of the diagnosis unit to be valid when the detecting operations in the open and close conditions are continuously performed.

Abstract: Air/fuel ratio responsive sensors are disposed upstream and downstream a catalyst to feedback-control air/fuel ratio based on the output of the upstream sensor while effecting sub-feedback control based on the downstream sensor. in a catalyst deterioration detecting processing, a proportional gain and an integral gain of a sub-feedback control are increased so that the downstream sensor output changes more remarkably in correspondence with deterioration of the catalyst. As the catalyst degrades, the amplitude and the frequency of the downstream sensor output increase. Catalyst deterioration is detected based on a ratio of downstream sensor output change to upstream sensor output change.

Abstract: An engine has an upstream-side catalyst and a downstream-side catalyst. First and second air-fuel ratio sensors are provided on the upstream and downstream sides of the upstream-side catalyst, respectively. A third air-fuel ratio sensor is provided on the downstream side of the downstream-side catalyst. The deteriorating state of the upstream-side catalyst is diagnosed during a period of time in which the third sensor is outputting a voltage in a predetermined range, and the air-fuel ratio is controlled to maintain the voltage output of the third sensor in the predetermined range during the period of time to diagnose the deteriorating state of the upstream-side catalyst.

Abstract: An engine has an upstream-side catalyst and a downstream-side catalyst. First and second air-fuel ratio sensors are provided on the upstream and downstream sides of the upstream-side catalyst, respectively. A third air-fuel ratio sensor is provided on the downstream side of the downstream-side catalyst. The deteriorating state of the upstream-side catalyst is diagnosed during a period of time in which the third sensor is outputting a voltage in a predetermined range, and the air-fuel ratio is controlled to maintain the voltage output of the third sensor in the predetermined range during the period of time to diagnose the deteriorating state of the upstream-side catalyst.

Abstract: Evaporated fuel gas from a canister is introduced into an intake path of an internal combustion engine. A target air-fuel ratio is changed to a value on the fuel rich side during the introduction of purge gas in accordance with the purge gas concentration. This change will restrict the shift of the air-fuel ratio toward the lean side, allowing a catalyst to operate at high purification efficiency even during the introduction of purge gas. The change may be varied in accordance with the volume of the evaporated fuel gas or a ratio of the evaporated fuel gas to the fuel supplied to the engine.

Abstract: An air-fuel ratio feedback control has an air-fuel ratio sensor and an oxygen sensor at an upstream side and a downstream side of a catalytic converter. A CPU feedback controls an air-fuel ratio detected by the air-fuel ratio sensor to a target air-fuel ratio, with a feedback gain being varied with an output of the oxygen sensor. The CPU further estimates an air-fuel ratio response speed based on a ratio between an exhaust gas flow amount and a capacity of the catalyst and determines a rate of deterioration of the catalyst. The CPU varies further the feedback gain based on the estimated response speed and the determined rate of deterioration.

Abstract: In an engine fuel supply system, the difference between the actual and target air-fuel ratios, an air-fuel ratio feedback correction coefficient and a learned correction amount are added as a parameter for diagnosing the fuel supply system. The diagnostic parameter is smoothed and the smoothed value is compared with a diagnosis reference value, thereby detecting a malfunction in the fuel supply system. Even if the learned correction amount is not updated, the malfunction in the fuel supply system can be promptly detected from the difference between the actual and target air-fuel ratios and the air-fuel ratio correction coefficient. The diagnosis reference value is determined variably in accordance with engine operating parameters such as the amount of intake air.