I have the same problem: I own a xsara picasso 1,8 (2002).
Suddenly and absolutelly randomly, the temp gauge goes red and the warning "STOP ENGINE TEMPERATURE" with the beep appears.
Even if I stop engine , lock the car and start over again, the singal does not stop and the ventilator is still on.
Only when I remove the #3 (yellow, 20A) fuse on the engine fuse box, 2nd line, the warning dissapears and the fan stops.
Does anyone have any idea what is wrong?
Could it be that " green coolant temperature sensor on the right side of the engine on the thermostat housing"?
Thanks in advance.