Systems and methods for detecting developing faults in a flow generator or ventilator during therapeutic use thereof are provided. The motor current may be measured to estimate the torque input by the motor, while the output torque from the impeller may be determined (e.g., as inferred from the motor control system model and/or by consulting a lookup table). One or more transducers may collect data useful in determining the input and output torques. A difference between the input (to the motor) torque and the output (from the impeller) torque may be calculated. The difference, optionally filtered using a low-pass filter to reduce noise, may be compared to a predetermined threshold once or over a period of time to detect gross failures and/or developing failures. Once a failure or developing failure is detected, a user may be alerted and/or the flow generator may be placed into a “service required” mode.