C1 Power Steering Faulty Connector

Sounds like a pain. I had something similar with my old Vauxhall Astra, although that was petrol, not diesel. Same symptoms though: vibrating steering, spongy feel, and the steering getting heavy. Turned out to be a dodgy connection on the steering pump. Have you checked the voltage getting to the pump? A multimeter might be your best bet.

Thanks. Could you elaborate on which specific connection you are referring to? As I understand it, the steering pump is connected to both power and a control module. Is there a specific area I should focus on when inspecting the connections and testing the voltage?

Sure. On the Astra it was the main power connector to the steering pump itself. It looked okay at first glance, but when I wiggled it, the voltage dropped right off. I'd check that first. Honestly though, if you're not 100% sure what you're doing with electrics, it might be worth getting a workshop to have a look. Steering is kinda important!

I took the vehicle to a trusted workshop. It was a faulty connector on the steering pump, just as suspected. The total cost, including diagnosis and repair, came to 95€. Thanks for pointing me in the right direction!