Air Fuel Ratio (AFR) imbalance between engine cylinders remains one of the most challenging problems in powertrain systems diagnostics. California Air Resources Board(CARB) has started imposing specific requirements on automotive companies since 2011 that required the integration of on-board diagnostics (OBD) monitor for the detection and reporting of this type of powertrain malfunction. In this paper, some methodologies of AFR cylinder imbalance monitoring are investigated and a novel approach is proposed that shows reliable detection capability compared to the other methods. The proposed method requires certain conditions during deceleration fuel shutoff events to intrusively reactivate the cylinders and determine the imbalance condition. The method was evaluated on a V6 3.7L engine in an experimental Lincoln MKZ vehicle. Vehicle results are shown and discussed.