When new, a sensor can produce a linear output for over 4 bar partial pressure of oxygen, and as the anode is consumed the linear output range drops, eventually to below the range of partial pressures which may be expected in service, at which stage it is no longer fit to control the system. The maximum output current eventually drops below the amount needed to indicate the full range of partial pressures expected in operation. This state is called "current-limited". When a current limited sensor can no longer reliably activate the control system at the upper set-point in a life support system, there is a severe risk of an excessive oxygen partial pressure occurring which will not be noticed, which can be life-threatening.