Where does the law stand on Mazda DPF removal?

Firmly with the filter on any Mazda that uses public roads. Without it the car breaches the Road Vehicles (Construction and Use) Regulations, every MOT from February 2014 has failed a diesel whose DPF is missing, and the driver faces a fine of up to £1,000, or £2,500 for a light goods vehicle. We carry out Mazda removals for off-road and motorsport vehicles only.

Why do Mazda 3 and Mazda 6 diesels block their DPFs?

Two things, usually together. Every regeneration leaves ash that cannot be burnt off, and the filter only regenerates properly on a steady 15 to 20 mile run at 40 to 50 mph, which a Mazda kept to town work rarely gets. The wrong grade of engine oil, lower-quality fuel and faulty exhaust sensors speed it up. Our Mazda 3 DPF page covers the warning signs.

Why is DPF work on a Mazda 6 harder than on other diesels?

Because of the software, not the exhaust. On the Mazda 6's RF7K and RF8G/RF8X engines the Denso engine ECU is heavily encrypted, so the DPF system has to be switched off properly inside it. Boxes that only trick the ECU leave that system active, and it keeps trying to regenerate, with smoke and poor economy as the result. Our Mazda 6 and Mazda 5 pages explain how we approach it.

Is a new DPF the answer for a blocked Mazda?

Rarely the first one. A genuine replacement is expensive and does nothing about whatever blocked the original, and cheap copy filters are known to block up quickly. On a road-going Mazda the better repair is to find the cause and have the existing filter professionally cleaned; our DPF explainer sets out why that works.