A method for identifying damaged diesel particulate filters having a multiplicity of generally axially-extending filter cells, said method comprising the steps of: directing a flow of compressed air from a first nozzle member into said axially-extending filter cells from a first end face of said diesel particulate filter; characterised by: simultaneously directing a flow of compressed air from a second nozzle member into said axially-extending filter cells from a second, opposite end face of said diesel particulate filter; moving said first and second nozzle members (30, 34) over substantially the entirety of said first and second end faces so that said flows of compressed air are directed into substantially all of said filter cells from opposite directions so as to dislodge and remove particulate material therefrom and wherein the locations of the nozzle members (30, 34) are always offset to avoid the air blasts counteracting one another; and visually observing flows of said particulate material dislodged and removed from said diesel particulate filter so as to identify damage to said filter cells thereof, as indicated by a visible forceful discharge of said particulate material simultaneously from both said first and second end faces of said diesel particulate filter. directing a flow of compressed air from a first nozzle member into said axially-extending filter cells from a first end face of said diesel particulate filter; characterised by: simultaneously directing a flow of compressed air from a second nozzle member into said axially-extending filter cells from a second, opposite end face of said diesel particulate filter; moving said first and second nozzle members (30, 34) over substantially the entirety of said first and second end faces so that said flows of compressed air are directed into substantially all of said filter cells from opposite directions so as to dislodge and remove particulate material therefrom and wherein the locations of the nozzle members (30, 34) are always offset to avoid the air blasts counteracting one another; and visually observing flows of said particulate material dislodged and removed from said diesel particulate filter so as to identify damage to said filter cells thereof, as indicated by a visible forceful discharge of said particulate material simultaneously from both said first and second end faces of said diesel particulate filter. simultaneously directing a flow of compressed air from a second nozzle member into said axially-extending filter cells from a second, opposite end face of said diesel particulate filter; moving said first and second nozzle members (30, 34) over substantially the entirety of said first and second end faces so that said flows of compressed air are directed into substantially all of said filter cells from opposite directions so as to dislodge and remove particulate material therefrom and wherein the locations of the nozzle members (30, 34) are always offset to avoid the air blasts counteracting one another; and visually observing flows of said particulate material dislodged and removed from said diesel particulate filter so as to identify damage to said filter cells thereof, as indicated by a visible forceful discharge of said particulate material simultaneously from both said first and second end faces of said diesel particulate filter.