Peugeot 106 weak battery

I had similar issues with my 106: weak starts and flickering lights. Have you checked the voltage across the battery terminals while the engine is running? A faulty battery sensor can cause low voltage and those symptoms.

No, I haven't checked the voltage. Where exactly is the battery sensor located? Is it difficult to get to?

It's often integrated into the battery terminal clamp itself, or very close to it, wired directly to the battery. I'm not sure about the diesel, but it's usually pretty accessible. However, given the symptoms and your suspicion, it's probably worth getting it diagnosed properly at a workshop.

Thanks for the advice. I took it to a shop; they confirmed it was the battery sensor. The repair cost me 315€, but it's fixed now.