S80 Brake Booster Pressure Sensor Error

That sounds familiar. I had a similar issue with my S80. Weak braking and a pressure sensor error. Did you happen to bleed the brakes yourself? On these cars, if the bleeding procedure isn't followed exactly, it can introduce air into the system, which throws off the sensor. It's like trying to measure the air pressure in a tire that's already flat – the sensor gets confused.

Yes, I did the brake bleed myself. Could you elaborate on what the correct procedure is? I suspect that's where I went wrong. I was using a generic guide I found online. What exactly did you do differently?

The correct procedure involves using a diagnostic tool to cycle the ABS module while bleeding. Otherwise, air can become trapped. Based on your description and the fact that you performed the bleed yourself without the proper tool, I'd strongly suggest you take it to a shop. They can properly bleed the system and confirm if the sensor itself is damaged. Continuing to drive with poor braking is not safe, and further attempts to fix it yourself without the right tools could lead to damage to other brake components.

Thanks for the insight! Taking it to a workshop makes sense. The brake system bleed was indeed the culprit. They re-bled the system correctly and replaced the faulty pressure sensor. Total cost was 90€. The braking is back to normal now.