Oxygen sensor deterioration-detecting system for internal combustion engines

An oxygen sensor deterioration-detecting system for an internal combustion engine having a plurality of groups of cylinders, an exhaust system having a plurality of exhaust passages extending from respective ones of said groups of cylinders, said exhaust passages having downstream end portions thereof cojoined into a confluent portion, and catalytic exhaust gas-purifying means arranged in said exhaust system, and a plurality of upstream oxygen sensors arranged in respective ones of said exhaust passages at locations upstream of said catalytic exhaust gas-purifying means, comprising:

• a single downstream oxygen sensor arranged in said confluent portion of said exhaust passages at a location downstream of said catalytic exhaust gas-purifying means, for detecting a mixed air-fuel ratio of exhaust gases emitted from said groups of cylinders;
• a plurality of air-fuel ratio control means responsive to outputs from said upstream oxygen sensors and an output from said downstream oxygen sensor, for determining air-fuel ratio control amounts for respective ones of said groups of cylinders, and for controlling air-fuel ratios of air-fuel mixtures supplied into said respective ones of said groups of cylinders by the use of said air-fuel ratio control amounts determined;
• and
• oxygen sensor deterioration-detecting means for detecting deterioration of each of said upstream oxygen sensors, based on an output from said each of said upstream oxygen sensors obtained by operation of a corresponding one of said air-fuel ratio control means.