A powertrain assembly includes an engine having at least one cylinder and at least one electric machine operatively connected to the engine. A motor speed sensor is operatively connected to and configured to obtain motor speed data of the electric machine. A controller is operatively connected to the motor speed sensor. The controller including a processor and tangible, non-transitory memory on which is recorded instructions for executing a method for detection of firing irregularities in the at least one cylinder. Execution of the instructions by the processor causes the controller to obtain the motor speed data at a predefined time interval from the motor speed sensor, until a predefined time window is reached. A fast Fourier transform of the motor speed data during the predefined time window is obtained. The controller is configured to control the engine based at least partially on the fast Fourier Transform.