171 Mazda 6 DPF issue 'Soot accumulation in DPF is too high'

How much and what kind of driving are you doing? You say "mostly motorway driving" and 6000km, but how long are the trips? And how often? And just to clarify, you got the first DPF error in November 2020, and then the next one was 14 months later (this month). The 6000km is since September 2020, not 2021. So less than 6000km in a year - which would be regarded as very low mileage for a diesel. A 171 that had 63,000km when you bought it in 2020 was averaging over 15,000km a year under the previous owner(s).

Multiple short trips without a long one to trigger a regen will cause the DPF to clog up.

That said, I had a previous car that was constantly getting a blocked DPF, despite me doing 50km round trips every day, and a 300km round trip every Sunday. Multiple garages (including one who traded on the notion of being dedicated DPF specialists) took a look at it, and none could figure out the problem.

Ah, ok, well that changes things alright. You look like you're doing enough mileage and the type of driving to keep a DPF clear.

Only thing I'd say is don't assume that the garage ran proper diagnostics on it. They might just have forced the regen without looking at the underlying issue. A quick scan for fault codes is not a proper diagnostic. I don't know about cleaning the DPF. I had it done in my old car, and it helped for about a month. That's not to say that it wouldn't work in another scenario. There's two residues in a DPF. There's the soot, which is the particles that are getting filtered out of the exhaust fumes. They eventually build up and require the regeneration process to burn them off. Then there's ash. The ash is the residue from the regeneration burning the soot. Over a longer period of time, the ash accumulation becomes a problem. And there's nothing the car can do to remove the ash. I think it's ash that DPF cleaning claims to remove. But I think modern cars are supposed to be smart enough to differentiate between soot and ash accumulation, and your error message is a soot one.

When I brought mine to a DPF specialist, they handed me about 200 pages of diagnostic logs, taken over a number of days (both driving and in the garage). They weren't able to pinpoint the reason that the car was refusing to regenerate. Previously, the DPF pressure sensor and sensor pipes had already been changed. EGR valve had been checked. A boost pipe leak had been identified and fixed. DPF had been removed and cleaned. This was over the course of a year of investigation. The car was over 10 years old and over 250,000 km on it. A new DPF was one option, and probably required since the DPF was probably damaged from being blocked, but the consensus was that there was something else happening that was causing the underlying issue. Because of all this, I ended up scrapping the car. But that was at the end of a fairly exhaustive (excuse the pun) process. You're not anywhere near there yet.

If I were you, first step would be to make sure that a full diagnostic and log is done to see exactly what is going on with the engine. Not just checking for error codes - looking at the various values of the engine's performance to make sure that they're all as they should be.

Oh, one other thing. Make sure the car is serviced properly with the correct oil. The oil is a very important part of the regeneration process, and an incorrect oil for the engine will hinder it. Be sure you're getting the exact spec oil the manufacturer requires for the specific engine.

Get dpf mapped out be best thing to do

Isn't the Mazda diesel engine a total nightmare anyway, apart from minor issues like a DPF problem?

If I remember rightly, there have been several post on here before, concerning Mazda diesel engines and DPF and engine oil issues. Their petrol engines on the other hand, seem to be pretty trouble free. Not too many good options all things considered, it is unfortunate that you did not do a bit of research before purchasing. Nowadays, there are many makes with DPF technology, and they all have the possibility to give DPF trouble, but Mazda DPF's seems to give problems over and beyond the norm, which are incredibly difficult to fix. Personally, if I could, I'd be getting rid of it.

If you read through the thread the OP corrected himself, the car was purchased 2021 not 2020 so 6k in 4 months

My 2011 Mazda had a similar problem, turns out the sensor was borked in the exhaust that was picking up the soot. Back then i had the DPF innards removed and remapped, but its a bad idea now with changes to the NCT looming.