Common Myths About Particulate Filters

Common Myths About Particulate Filters

A diesel particulate filter (DPF) is an integral component of the exhaust system of a diesel-powered vehicle that is a legal requirement on modern cars. The idea is that they reduce exhaust particulate emissions by trapping them within the filter. That means cleaner air. They work and get with the job effectively but they do not have infinite capacity.


Diesel particulate filters clean themselves in one of three ways and owners should be guided by the vehicle’s handbook. It’s a straightforward process and, with the recommendations correctly applied, the DPF will give long service. This however will depend upon how the vehicle is used and maintained. A clogged DPF will adversely affect vehicle performance and require cleaning by a car servicing garage. It’s not a DIY job.

It’s become something of an urban myth that DPF filters are always to blame when cars do not perform as they should. Certainly, diesel motors that are only used locally and for short trips will likely suffer more from the DPF being clogged but it can’t necessarily be assumed that any apparent faults with the car are down to DPF failure. Here are some examples:

Poor Fuel Economy

A clogged filter will certainly reduce fuel mileage as the vehicle works harder to compensate, but the fault may lie with lack of regular car servicing. Something as simple as a clogged air filter will reduce economy, as will faulty fuel injectors.

Under-Inflated Tyres

If questioned closely, many motorists will admit they do not check their tyre pressures as often as they should. Not only will tyres wear quickly when the pressures are low, they will also not perform as they should. This is a known cause of excessive fuel use and is simplicity itself to rectify.

Fouled Spark Plugs

Lack of servicing brings with it a whole list of potential issues. Spark plugs are a routine component and they usually last a considerable time but they do deteriorate; carbon fouling for example can be caused by a fuel/air mix that is too rich.

Don’t Assume

Essentially what it boils down to is correct servicing. A diesel particulate filter can cause any of the above problems but it should not be the first thing to consider. Some systems use the engine control unit to increase exhaust temperature to burn off soot build-up; others will require being run at speed, on a motorway say, for around a half-hour. The filter will then oxidise particulates and self-clean. This is why cars that are only used around town suffer more DPF problems; they don’t do extended runs or the cleaning process is interrupted by stop-start driving and may well be why the DPF warning light comes on.

Replacing a DPF is an expensive job so it pays not to jump to conclusions. Poor vehicle performance can be attributed to many things so don’t fall into the trap of believing the old myths that a DPF is the root of all diesel motoring ills. Use them properly and they will give good service.

So before attempting a DPF clean ask your regular servicing garage to check. They can quickly diagnose issues by plugging into the car’s diagnostic port. Remember too that a diesel particulate filter is a legal requirement and will be checked come MOT time. Don’t even think about taking it off!

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