Some exemplary embodiments include methods of operating a hybrid powertrain system including an engine and a motor/generator. One exemplary method includes sensing a characteristic of the motor/generator, determining a first net torque of the engine based upon a model, determining a second net torque of the engine based upon the characteristic of the motor/generator, and diagnosing the system based upon the first net torque and the second net torque. Further exemplary embodiments include hybrid powertrain methods, hybrid powertrain systems, and articles of manufacture configured to store computer executable instructions for hybrid powertrains. Further embodiments, forms, objects, features, advantages, aspects, and benefits shall become apparent from the following description and drawings.