ECU calculates variations of air-fuel ratios of respective cylinders and
variations of air-fuel ratios of all the cylinders in variable intake
valve lift devices of two cylinder groups. Presence and absence of an
abnormality is determined on the basis of whether ratios of variations of
air-fuel ratios of respective cylinders to variations of air-fuel ratios
of all the cylinders are less than an abnormality threshold. Whether
which of variable intake valve lift devices of the cylinder groups is
abnormal is determined from control modes of the variable intake valve
lift devices of the respective cylinder groups, and the relationship
between that cylinder group, to which a cylinder being maximum in
air-fuel ratio belongs, and cylinder group, to which a cylinder being
minimum in air-fuel ratio belongs.