Peugeot 806 radar sensor fault

Possible. The Peugeot 806's radar sensor, especially on a 2002 model, is sensitive to dirt. Have you checked the sensor's condition? Is it visibly obstructed or damaged? These systems are interconnected; a fault in one sensor can cascade into multiple system failures. Has any work recently been done on the front of the vehicle, that may have disturbed the sensor?

I haven't checked the sensor yet, but that's next. Where is the radar sensor located on the 806? Is it easily accessible, or will I need to disassemble the front end to get to it?

It's usually behind the front bumper, near the fog light. Access can be tricky. Before dismantling anything, check the wiring harness and connector. Corrosion is common. If you're not comfortable with electronics, consider a workshop, especially given the potential for cascading errors. Diagnostic tools might be needed.

I checked it out and it was caked in mud! A cleaning seems to have worked; the error is gone and all systems are back to normal. Cost me 45€ for the cleaner at the workshop, but now I know! Thanks for pointing me in the right direction!