Oxygen is one of the fuels of the cell so the more oxygen there is at the reaction surface, the more electrical current is generated. The chemistry sets the voltage and the oxygen concentration controls the electric current output. If an electrical load is connected across the cell it can draw up to this current but if the cell is overloaded the voltage will drop. When the lead electrode has been substantially oxidised, the maximum current that the cell can produce will drop, and eventually linearity of output voltage to partial pressure of oxygen at the reactive surface will fail within the required range of measurement, and the cell will no longer be accurate.