The Plague Monks pushing the Plague Furnace only suffer a wound from the Great Censer's fumes (see Enshrouded by Fog) on a roll of 6. If the Plague Priest has a Plague Censer, to work out models affected treat the base of the entire Plague Furnace as being the Plague Priest's base. Note that the Great Censer and a Plague Censer are different enough that a model affected by both needs to take two Toughness tests.