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.

"Exhaust Gas Recirculation" valve..... What is it and why is it fitted? Good question, and commonly mis-understood..... To answer this question, we need a little science, specifically Chemistry... Primarily, EGR valves combat Nitrogen Oxide emissions (NOx). NOx emission is directly connected to combustion temperature, and as combustion temperatures increase, so do NOx emissions, owing to the presence of Nitrogen and Oxygen in the atmosphere and the chemical reaction that occurs when you combust them. Simple..... but, there is another benefit to the humble EGR valve as well..... An engine relies on a cylinder, or multiple cylinders, with a piston moving up and down inside each cylinder, to fill with air and fuel at a specific ratio which, when ignited, causes expansion of the air, forcing the piston back down again. This is the basic operational principle of an internal combustion engine. Rotary (Wankel) engines don't have pistons but the principle remains the same. Diesel burns much less cleanly than petrol and is inherently more polluting, but in either case, by burning a fossil fuel, whether it be petrol, diesel or LPG, carbon (soot) is produced as well as various gasses such as Carbon Monoxide, Nitrogen Oxide etc. The air in our atmosphere is around 78% Nitrogen and 21% Oxygen. When exposed to extremely high temperatures, typically within the combustion process, the normally inert Nitrogen reacts with the oxygen and produces Nitrogen Oxide (NOx). The principle of an EGR valve is to recirculate some of the already burnt gasses back into the cylinders during times when you aren't demanding power (ie - cruising, light throttle or slowing down etc. The mechanical process I won't bore you with just yet), which results in less space for fresh, clean air to occupy. With less fresh air in the cylinder, and with that less Nitrogen and Oxygen, the quantity of fuel needed becomes less. The combustion process thus generates less heat, but ultimately fewer NOx particles as well. As for the 'added benefit of an EGR valve, think logically for a moment about carbon and what happens when you burn it. Carbon (Soot) is the inevitable consequence of burning a fossil fuel, so if we can burn it afterwards and convert it into something less harmful, there is an obvious benefit to doing so. In essence, that's what a DPF does, but that's a topic for another post..... Carbon will burn at temperatures above 400° celcius. It's the same principle used in a self-cleaning domestic oven..... By sending already burnt exhaust gasses, containing soot, back into the engine and burning it within the combustion cycle of each cylinder, the process will chemically convert the Carbon into Carbon Dioxide. The gas consumed by plants in order to photosynthesise. Yes, it does increase the overall CO2 emitted but it does effectively reduce the soot emitted, which could be argued is more harmful to human respiratory systems. This process is so efficient that the overall carbon emission of an engine fitted with an EGR valve is somewhere around 70% lower than one without, so it's not only to reduce NOx emissions. An EGR valve plays a massive role in reducing the harmful emissions from an engine, more-so in the case of a diesel! Now, here's the caveat... In my opinion, an EGR valve causes so many issues we're all familiar with..... Engines from the dawn of time have incorporated a 'crankcase breather' system. Often called a PCV or "Positive Crankcase Ventilation" system. To explain......... As pistons move up and down within their cylinders, they create a draught, or wind, within the engine itself. That wind causes a lot of pressure within the engine, so it must be vented somewhere where it can't cause undue stresses on the engine internals, especially at higher rpm's. Because it naturally contains oil vapours from the engine oil itself, venting it to atmosphere is a big no-no, so instead it is vented into the inlet where it can be drawn into the cylinders and burnt through the engines natural combustion process. As a result, it's perfectly normal to find some oil residue in the engines inlet pipes, intercooler, inlet manifold etc. This is perfectly fine and has been done for decades without a problem..... Until the inclusion of the EGR valve... I cannot fathom why manufacturers and engine developers continue to develop engines and emissions systems which mix exhaust gasses, containing soot/carbon via the EGR valve with oil vapour from the PCV system, and think it's a good idea! As has been proven time and time again, the combination of oil vapour and soot congeals to form an ever increasing layer of gunk in the inlet system which over time restricts airflow and causes the EGR valve to become so constricted that it either cannot allow enough air through into the engine, or it fails completely, leading to the reason many people chose to remove or blank the EGR as a more cost effective remedy which, not only has it's own legal implications but also, means the environmental benefits of the EGR are also removed causing the exact polution to our atmosphere it was designed to prevent! The solution, in my opinion, is simple.... F(http://simple.to)it an oil vapour catch-can and make draining it part of a standard service. A simple float sensor will indicate when the catch-can is becoming full, and a drain tap on the bottom will allow easy emptying of the oil it's accumulated. It's such a simple and effective concept! This way the soot can pass straight through the inlet and will never build up within the intake system, and the PCV system can also do its job at the same time without causing any issues to your engine at all. For those who choose to remove or blank the EGR valve, I completely understand why. Manufacturers not only make the system self-failing, but in many cases extremely difficult to service. Utilising a catch-can system would make it fool-proof, but instead they continue to combine oil vapour and soot, often fitting the EGR in an inaccessible place, which means its often extremely difficult to remove the valve in order to clean and refit. The whole system is thus designed to fail! If, however, you can forgive all its foibles, the environmental benefit of having an operational EGR valve system is huge! A fully functioning EGR valve has a profoundly beneficial effect of the emissions from an engine, specifically a diesel engine, and is probably the one, singlemost beneficial exhaust gas treatment devices ever developed!