Recently emissions regulations are being strengthened. An air-fuel ratio cylinder imbalance causes emissions to increase due to universal exhaust gas oxygen (UEGO) sensor error or exhaust gas oxygen (EGO) sensor error. Various methods of reducing an air-fuel ratio cylinder imbalance have been developed. It is preferable for a control system to operate over a wide range of conditions. Our target is to expand the operating conditions from idling to high load conditions.Our approach is to use both an UEGO sensor and a crank angle sensor. A two-revolution frequency component calculated from the UEGO sensor output signal and angular acceleration calculated from the crank angle sensor output signal are used to identify the cylinder where the air-fuel ratio error occurs. The detection result using the UEGO sensor precedes the detection result using the crank angle sensor when the engine is operated under a high load because the detection accuracy of the method using the UEGO sensor is higher under these conditions. On the other hand, the result using the crank angle sensor precedes the result using the UEGO sensor when the engine is operated at low engine speeds because the detection accuracy using the crank angle sensor is higher under these conditions. Consequently, it is possible to expand the control operating conditions.Experimental results using a real car showed that the developed control method operated under most of the conditions in the Japanese test cycle (JC08 Hot) and reduced NOx emissions by 90% compared with a car without the developed control method where an air-fuel ratio cylinder imbalance occurred.