Air-fuel ratio cylinder imbalance is a condition where the air-fuel ratio in one or more cylinders is different than the other cylinders. When air-fuel ratio cylinder imbalance occurs in one or more cylinders the fuel delivery system is unable to maintain a proper vehicle emission level.It is required for on-board diagnostics to detect air-fuel ratio cylinder imbalance levels which cause the fuel delivery system to be unable to maintain vehicle emissions at or below 1.5 times of any of the applicable FTP (Federal Test Procedure) emission standards. Currently California Air Resources Board (CARB) only requires on-board diagnostics to detect exhaust bank specific air-fuel ratio cylinder imbalance. In the near future CARB will require on-board diagnostics to detect cylinder specific air-fuel ratio cylinder imbalance. This paper presents a non-intrusive approach on determining single cylinder air-fuel ratio imbalance identification (ID) with an oxygen sensor. It employs digital filtering technique to extract the air-fuel ratio cylinder imbalance signals from existing upstream oxygen sensor voltage signals on single or dual exhaust bank engines. The filtered oxygen sensor voltage signals are processed with mathematical manipulation in order to determine the imbalanced cylinder. The oxygen sensor can be a binary or wide range type, however only the binary type is considered here. Different filter coefficients should be used for different types of oxygen sensors.