Degradation diagnosis device and degradation diagnosis method for exhaust gas sensor

Abstract

Provided is a device for performing degradation diagnosis of a downstream-side exhaust gas sensor. When a diagnosis condition is met, an intake-air quantity is successively accumulated to obtain an intake-air-quantity accumulation value, an intake-air-quantity average value is obtained as an average of the intake-air quantity from the start of diagnosis, and, on the basis of this intake-air-quantity average value, an intake-air-quantity accumulation threshold value is set. When the intake-air-quantity accumulation value reaches the intake-air-quantity accumulation threshold value, the diagnosis is terminated, and an output reversal number of the downstream-side exhaust gas sensor during the diagnosis period is compared with a threshold value to judge whether the sensor is normal or degraded. By setting the intake-air-quantity accumulation threshold value on the basis of the intake-air-quantity average value, influence from the flow velocity of the exhaust gas passing through a catalytic device and the downstream-side exhaust gas sensor is removed.