Fiat 500 diesel generator issue

I had similar symptoms with my Fiat 500, though I drive a petrol version. Reduced power, flickering lights... it sounds suspiciously like a faulty generator to me, too. Have you checked the voltage across the battery terminals while the engine is running? If it's significantly below 14V, especially with some electrical load (headlights, fan), then the generator is probably not charging the battery properly. Also, have you experienced any engine misfires before the other symptoms?

Thank you for your quick response. I haven't checked the voltage like that, but it's a good idea. Would a defective generator also cause the starting problems? I was thinking it might be a completely different issue. Is it expensive to fix? It does sound expensive.

A failing generator definitely can cause starting problems. If the battery isn't being charged correctly, it slowly drains down, making it harder and harder to crank the engine. So it is all connected. As for the costs, I am afraid I cannot give you definite information about the costs, but based on what you say, I would consider going to a workshop. Then at least you know what's what.

Thanks a lot for your advice! I took it to a workshop, and indeed, the generator was defective. The repair cost me 960€, but now the car runs smoothly again. Your insights were incredibly helpful in pointing me in the right direction!