A closed loop system or semi-closed loop system for infusing insulin using sensor values comprising: a first glucose sensor for location at a first site to generate a first blood glucose reading; a second glucose sensor for location at a second site to generate a second blood glucose reading; a controller 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 of the absolute error values of the first and second errors to a threshold; determining a failing sensor (800, 850) exists if the sum of the absolute error values exceed the threshold; determining whether the first glucose sensor or second glucose sensor has the least error in the sensor signal if the sum of the absolute values does not exceed the threshold; calculating (1680, 1690) a reported blood glucose value based on the first predicted value if the first error is less than or equal to the second error; and calculating (1680, 1700) a reported blood glucose value based on the second predicted value if the first error is greater than the second error; and an infusion pump configured to deliver insulin based on the reported blood glucose value. a first glucose sensor for location at a first site to generate a first blood glucose reading; a second glucose sensor for location at a second site to generate a second blood glucose reading; a controller 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 of the absolute error values of the first and second errors to a threshold; determining a failing sensor (800, 850) exists if the sum of the absolute error values exceed the threshold; determining whether the first glucose sensor or second glucose sensor has the least error in the sensor signal if the sum of the absolute values does not exceed the threshold; calculating (1680, 1690) a reported blood glucose value based on the first predicted value if the first error is less than or equal to the second error; and calculating (1680, 1700) a reported blood glucose value based on the second predicted value if the first error is greater than the second error; and an infusion pump configured to deliver insulin based on the reported blood glucose value. 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 of the absolute error values of the first and second errors to a threshold; determining a failing sensor (800, 850) exists if the sum of the absolute error values exceed the threshold; determining whether the first glucose sensor or second glucose sensor has the least error in the sensor signal if the sum of the absolute values does not exceed the threshold; calculating (1680, 1690) a reported blood glucose value based on the first predicted value if the first error is less than or equal to the second error; and calculating (1680, 1700) a reported blood glucose value based on the second predicted value if the first error is greater than the second error; and an infusion pump configured to deliver insulin based on the reported blood glucose value.