A closed loop system or semi-closed loop system for infusing insulin using sensor values comprises； a first glucose sensor (800) for location at a first site to generate a first blood glucose reading (1610)； a second glucose sensor (850) for location at a second site to generate a second blood glucose reading (1620)； a controller (12) configured to perform the steps of: deriving a first predictive value to the first glucose reading using the second glucose reading as an input； deriving a second predictive value to the second glucose reading using the first glucose reading as an input； determining a first error between the first predictive value and the first glucose reading； determining a second error between the second predictive value and the second glucose reading； comparing a sum (1650) of the absolute error values of the first and second errors to a threshold (1660)； determining a failing sensor (800, 850) exists if the sum of the absolute error values exceed the threshold； determining whether the first glucose sensor (800) or second glucose sensor (850) has the least error in the sensor signal if the sum of the absolute values does not exceed the threshold； and calculating (1680, 1690, 1700) a reported blood glucose value based on the glucose sensor (800, 850) having the least error in the sensor signal.
