A diesel particulate filter is a device designed to remove diesel particulate matter or soot from the exhaust gas of a diesel engine. As the gases pass through the filter, the filter will begin to get clogged with the soot that it has trapped. (Soot is a by-product of incomplete combustion - when the mixture of air and fuel that powers the engine fails to completely ignite). As the temperature of the exhaust gets higher, the residue heats up and turns to ash which is then expelled.