Diesel particulate filters trap soot from the exhaust and ash from motor oil. Most soot is burned off in the course of a truck's operations, but ash stays in the filter's honeycomb substrate and is removed through periodic servicing.


Exhaust in highway trucks is usually hot enough to burn off most soot in a process called "passive regeneration." However, those involved in stop-and-go operations or that idle a lot don't get their exhaust hot enough. The latter must "actively" burn out soot from the substrate by injecting extra fuel just upstream of an oxygen catalyst or by plugging in an electric heater when parked.

Active removal of soot through on-board regeneration can occur one or more times a day, depending on its type of operation, manufacturers say. Often it goes unnoticed by the driver, who may or may not see the indicator light in the instrument panel. But sometimes a warning gets more insistent through the light indicators and the driver must stop and initiate an active regeneration.

Ash from motor oil stays in the filter's substrate and must be periodically blown or washed out. This is done by removing the DPF from the truck and placing it on a special machine. It sends compressed air through the substrate in a series of measured puffs. Detroit Diesel's filters are pressure-washed with de-ionized water, then dried.

Most filters seen by Cleaire Advanced Emissions Controls, which air-cleans DPFs it has previously made and sold, collect about 200 grams (7 ounces) of ash. Some come in with as much as 600 grams (21 ounces).

Blowing or washing out of ash must be done every 50,000 miles or once a year. This, too, is indicated by a warning light. Large fleets have purchased machines to do the cleaning themselves, but most owners take filters or entire trucks to dealers and independent shops for cleaning.

Some owner-operators running out of the Port of Oakland, Calif., have used state money to retrofit their older trucks with DPFs, only to discover they need frequent removal and cleaning. So it's important to ask upfront what the cleaning requirements will be, how long they will take and how much they will cost.

Sometimes an engine produces extra soot or spits out unburned fuel and burned motor oil. These, too, are captured by the DPF, and usually it can't burn them out through normal processes. So occasionally the substrate must be removed from the DPF, then heated and baked in a special machine to burn out the crud. About 10% of filters brought in for servicing need this, manufacturers say.