A system and method for detecting input phase loss in an adjustable speed drive (ASD) includes an input unit to detect operating data from the ASD. The operating data includes a DC link current of the ASD. The system also includes a state observer that is adapted to receive the operating data from the input unit and extract a DC link capacitor current of the ASD using the DC link current. The system also includes a controller programmed to compare the extracted DC link capacitor current to a predetermined fault range and generate a fault indication of an input phase loss if the extracted DC link capacitor current is within the predefined fault range. The controller is also programmed to calculate an estimated lifespan of the DC link capacitor based on the extracted DC link capacitor current.