Mercedes C Klasse diesel cold start problem

Whoa, sounds like a drag. If your HVAC system is throwing codes and you are getting cold air when you should not, plus the blower motor is acting up, it is super likely your PTC heater is fried. I had a similar thing happen on my old car – the heat not working right was the first sign, then the rough starts. Have you checked the fuses related to the auxiliary heater? Sometimes it is just a simple fix, fingers crossed.

Fuses are fine. So are you saying the PTC heater being faulty could actually cause rough running during cold starts? I thought it was only for heating the cabin.

Yep, absolutely. On these newer diesels, the PTC heater helps the engine reach operating temperature faster, especially in cold weather. If it is not working, the engine stays cold longer, messes with the fuel mixture, and leads to poor running and higher fuel consumption. If the fuses are ok and you are sure it is the PTC, sadly you are probably looking at replacing it. If you are not comfortable doing that, I would suggest visiting a workshop.

Thanks for the confirmation. Replacing the PTC heater fixed the issue. It cost me around 350€. Engine runs smooth now, and the fuel consumption is back to normal.