C3 brake booster pressure sensor fault

I had a similar issue with my C3. The symptoms you describe are consistent with a faulty brake booster pressure sensor. It's likely the sensor itself needs replacing.

Thanks for the reply! Was it definitely the sensor, then? And did it cause the poor braking performance as well as the error? How much did it set you back, if you don't mind me asking?

Yes, in my case, the diagnostic pointed directly to the brake booster pressure sensor. The error definitely impacted braking performance. Given your description, a visit to a workshop for a proper diagnostic and sensor replacement would be prudent.

Okay, thanks so much for the advice! I took it to a local garage, and you were spot on! They confirmed it was the brake booster pressure sensor. Apparently, it was likely damaged when the previous owner had the brake system bled and they got some air in the system. Cost me 90€ all in for the part and labour. Brakes are back to normal now, and the error message is gone! Cheers!