Sulfur inhibition in catalysts is not completely reversible. Although conversion
efficiency will always improve with return to reduced sulfur levels, the efficiency of
the catalyst does not usually fully return to its original state after desulfurization. In
tests using 60 ppm sulfur fuel followed by a single use of 930 ppm sulfur fuel, HC
emissions tripled from 0.04 g/mile to 0.12 g/mile. With a return to low sulfur fuel,
emissions dropped again to 0.07 g/mile but fuel-rich operation (resulting in high
exhaust temperatures) was required to regenerate the catalyst fully and return to
original emissions levels.