Like them or loathe them, they're here to stay. Every diesel engined vehicle from around 2008/9 onwards has one, but they're very often mis-understood and many of the problems we hear about are not directly the fault of the DPF.
DPF's rarely fail. They either fill with ash as a natural byproduct of what they're designed to do, or they fill with soot. If they are full of soot, it's because it's not able to regenerate properly, and there will be an underlying reason not linked to the DPF itself, such as an intake air leak causing an incorrect fuel/air ratio, or a faulty sensor etc. In many cases the DPF is wrongly blamed because garages/mechanics fail to correctly diagnose the root cause of why the DPF has a problem, and fitting a new DPF won't resolve it. All that's likely to happen is the DPF will eventually fill with soot again within a few weeks because the root cause wasn't resolved, and you'll be back to square one.... minus the money wasted on a new DPF....
Let's start with the what?.......
In essence, a DPF is a just a filter. It allows exhaust gasses to pass through it but traps larger soot particles and prevents them from exiting the exhaust and polluting the atmosphere.
Why?..... Well, primarily because European Emissions Regulations dictate that diesel soot emissions need to be under a specific level for Euro 4 compliance onwards. The other reason is the obvious one..... So that we don't breathe it in.....
And then the how?.......
Soot/Carbon is a natural byproduct created by burning fossil fuels, and diesel is the most filthy of them all.....
Hold a piece of paper over a candle and it will turn black from the soot. The same is emitted by an internal combustion engine, and diesel will emit the most soot of any engine. Frankly, it's filthy!
To further reduce the soot emissions of an engine, the European Emissions regulations were tightened in the UK in January 2005, commonly known as Euro 4, and subsequently many engines that didn't meet the requirement needed DPF's to be fitted. We're now on Euro 6 and the DPF is still a mainstay component.
So how does the DPF work? In much the same way as a self-cleaning domestic oven..... It heats up and burns the carbon into Carbon Monoxide, Carbon Dioxide and, with some systems, Nitrogen Oxide.
Ash also slowly accumulates within the DPF over many thousands of miles as a natural byproduct of burning carbon. The ash particles are small enough to pass through the filter, but some does accumulate over time, reducing the space left to collect soot leading to it filling up quicker and needing more and more frequent regenerations. Ash cannot be burned, nor can it be dissolved, so will eventually require the DPF to be removed and mechanically cleaned or replaced.