The conductivity of partially sintered doped ZnO varistor pellets was shown to increase via two mechanisms with introduction of or CO/Ar atmospheres. Reducing gas extracted chemisorbed oxygen, which in turn reduced current‐opposing potentials which had formed at grain boundaries. This mechanism was gas concentration independent. After more extended times, the conductivity showed a second increase as extraction of lattice oxygen near the grain boundaries further lowered the grain boundary Schottky barriers. During this time period, the time rates of change of current through the sensor were shown to increase with concentration (1 to 20%), when the compact sustained continuous porosity, and was heated between 300 and 500°C. This resulted from the rate of extraction of lattice oxygen being limited by the availability of gas at grain boundary neck surfaces. CO/Ar atmospheres were less effective at removing chemisorbed and lattice oxygen, and current slope sensitivity to CO concentration was not demonstrated.