A method for fault-tolerant coasting control of a powertrain system having an engine and a first energy storage system (ESS) includes receiving a real impedance value of the first ESS from a frequency analyzer device at a calibrated frequency while the engine is running, and comparing the real impedance value to a calibrated impedance. A coasting maneuver is enabled allowing the engine to turn off above a threshold speed when the real impedance value is less than the calibrated impedance. The method may include starting the engine using a second ESS in parallel with the first ESS to exit the coasting maneuver when the real impedance value exceeds the calibrated impedance. Subsequent execution of the coasting maneuver may be prevented as long as the real impedance value exceeds the calibrated impedance. A powertrain system includes the engine, starter motor, rechargeable ESS, frequency analyzer, and controller.