Diesel CLK power loss and smoke issue

Sounds like a DPF issue. Reduced power and smoke are classic symptoms. Have you checked for error codes? A faulty backpressure sensor could also be the culprit, preventing the DPF from regenerating. A diagnostic scan should point you in the right direction. It might be worth trying a DPF cleaning before replacing the filter, depending on how clogged it is.

Where would one typically find these error codes? Can I access them myself, or is it best left to a professional with diagnostic equipment? Also, what are the potential risks associated with DPF cleaning, and how does one determine if it's too late for that?

You can get a basic OBD2 scanner online or from most auto parts stores to check the error codes yourself. You just plug it into the port under the dash and follow the instructions. Regarding DPF cleaning, sometimes you can clear it with a plunger, but if the blockage is too severe, you need to replace the pipe. If the DPF is severely damaged, cleaning might not be effective and could even cause further issues. A mechanic can assess the condition of the DPF and advise you on the best course of action. Given the symptoms, a trip to the workshop is probably the safest bet to avoid further damage.

I took it to a trusted workshop, and after their assessment, it was indeed a defective diesel particulate filter. The filter replacement, including labor, came to 1550€. I'm now back on the road with the engine running smoothly again. Thanks for pointing me in the right direction.