SMAX EGR Valve Failure

That sounds like a textbook case of EGR valve failure, especially with those symptoms in conjunction. Think of the EGR valve as a gatekeeper, carefully controlling the amount of exhaust gas recirculated back into the intake. When it malfunctions, that careful balance is disrupted. However, before condemning the EGR valve outright, have you inspected the EGR cooler for any blockages or leaks? A faulty cooler could also contribute to those symptoms. Also, ensure there are no leaks between the EGR valve and intake manifold.

Interesting thought about the EGR cooler. I hadn't considered that specifically. I've been so focused on the valve itself. Whereabouts is the EGR cooler typically located on this engine? And is there a relatively straightforward way to check it for blockages without dismantling half the engine bay?

The cooler is typically mounted close to the EGR valve, often integrated into the intake manifold or exhaust manifold assembly. Visual inspection is your friend here - look for any signs of leaking coolant or excessive carbon buildup. You might also be able to disconnect the hoses connected to the cooler and attempt to blow compressed air through it to check for blockages, though you'll need to ensure all the downstream connections are disconnected or damage could occur. Given the complexity of modern diesel systems and the potential for further complications, I'd still recommend taking it to a trusted workshop. The cost of further unintended damages can exceed the cost of professional repair.

Thanks for the advice. I had it looked at by a shop after getting the same recommendation by a friend. It turned out you were spot on with the EGR valve - that was indeed the culprit. Replacement including labor came to 355€. I appreciate your input!