If you are talking diesel I would highly recommend adding some Archoil, 6400-D to clean the system out and then the AR6200 I think it is as a fuel additive.
It really is pretty incredible stuff. I have run it on all my diesels now for the last few years with some pretty amazing results.
I ran a 530d which I bought with an EML on at 60k miles, it was a gunked up EGR valve and manifold, which was also filling the EGR up far too quickly. I replaced the EGR for a new one and cleaned the manifold and then ran the car to 116k miles using Archoil.
Just before I sold it, I did take off the EGR to have a look, and where they are normally starting to gunk up pretty badly, this was clean, literally had a layer of soot dust on it which wiped off with a cloth.
I have just helped a mate out with his E350cdi, EML light, saying DPF ash and soot levels too high at 100k miles. Mercedes changed the EGR, DPF pressure sensor, and it was fine, for another tankful and back to square one.
The car was not doing a regen.
They wanted to put a new DPF on. That is great, it will sort it.....for a few thousand miles.
I said we need to start from the beginning and work out why it is getting blocked.
First thing, change the oil. You need to know 100% you have a decent low ash oil, makes a hell of a difference to how quickly your DPF fills.
Secondly, stop using Sainsburrys diesel, it is terrible for these cars. I did some pretty thorough testing on my E350cdi when I had it with Sainsbury diesel vs Shell (regular) and Shell with some Archoil. On Sainsbury fuel it would try and regen nearly every tank, that tells you something.
On Shell it was every 2-3 tanks.
Using Shell and Archoil it is every 4-6 tanks. In fact using Tesco or Esso and Archoil was the same too.
So, we swapped the oil over, filled up with Shell V Power (similar additive as Archoil, just not as strong) and within 30 miles his car did a regen, you could see the crap coming out.
Using a scanner we kept an eye on it, and some 3 tanks later the car is running great, his average has gone from 36mpg to 44mpg doing the same commute, and the DPF is still showing as no increase in soot or ash levels.
I had the same issue with Sainsbury petrol too in the 2012 E350, the EML would come on, scan it and it would say "Mis fuelled or unknown additive found in fuel." It would say there was also an issue with the NOX sensor. Mercedes have had so many issues with NOX sensors and have been replacing them, the reality is they are set too sensitive, and with some of the fuels it would throw and EML.
Decent, clean oil and decent diesel is so, so, so important for these cars, especially if you don't want issues with EGR, DPF and Nox sensors etc.
I have sorted a few cars out now that had issues with EGRs and DPFs with these simple steps, just did a Ford Transit, with 190k miles on it, was told new DPF and another vaporiser (they changed this 20k miles ago) needed. Nonsense.
We got it sorted for £50 worth or cleaners, and EML that Ford have not managed to get it and keep throwing parts at.
The additives simply help the car to burn the fuel more cleanly, just think of an older diesel with no EGR and DPF, old cheap fuel and old dirt oil and you see a cloud of black smoke when you put your foot down, we just don't see it now as that is in your dpf. No wonder they get so blocked.
Change oil every 8-10k and use decent fuel or an additive and you will have no problems.