A method for diagnosing failure of a cold start E/M reduction system confirms the change in the torque reserve compared to the torque reserve confirmed for the ignition timing upon cold start by any one of an operation of an electric load device, an operation of an alternator, an operation of a purge valve, an operation of an air conditioner, a shift state of a transmission, and a coolant temperature state of an engine, and confirms the failure or normality of any one of the electric load device, the alternator, the purge valve, and the air conditioner is normal or failed using the change in the torque reserve to apply the confirmed result to determine whether the cold start E/M reduction system is abnormal, securing failure diagnosis accuracy for the components for the cold start E/M reduction.