Inter-cylinder air-fuel ratio variation abnormality detection apparatus for multicylinder internal combustion engine

An inter-cylinder air-fuel ratio variation abnormality detection apparatus for a multicylinder internal combustion engine, the apparatus being configured to calculate a first parameter correlated with a degree of a variation in output from an air-fuel ratio sensor installed in an exhaust passage common to a plurality of cylinders and comparing the calculated first parameter with a predetermined determination value to detect variation abnormality in air-fuel ratio among cylinders, the apparatus being configured to carry out:

• (A) a step of calculating the first parameter;
• (B) a step of determining a possible range of a second parameter representing a degree of a variation in air-fuel ratio among the cylinders based on the calculated first parameter;
• (C) a step of calculating the first parameter with an air-fuel ratio of a predetermined cylinder forcibly changed;
• (D) a step of calculating a difference between the unchanged first parameter and the forcibly changed first parameter;
• (E) a step of determining a first characteristic representing a relation between the second parameter and the difference based on the possible range of the second parameter and the difference;
• and
• (F) a step of correcting one of the determination value and the first parameter calculated in the step (A) based on inclination of the determined first characteristic.