Citroen Xsara Picasso overheating reading on dash

i had the prolem with a picasso. he temp gauge sudden went from half way to full without climbing. this was because the fan wasnt working and the car sensed the fault with the fan and automaticaly went to overheating on temp gauge and dash display as a precaution. check the fan isnt seized and check the fan switch is opeating correctly as i had to replace both as they were faulty.

I have a same problem: The car shows overheat on dash, the ventilator starts and it does not stop even if I switch the engine off. This might drive to battery depletion, so when this situation occurs I remove the fuse of the ventilation system and I replace it. After this, I switch on the engine and everything is OK. Untill next time, not periodically, totally unexpected, no matter if the car is working or it is parked.
This means that some nigths the ventilator starts by its own and consumes all the battery power, so in the morning the car has run out of battery and I have to get it recharged!!!
Does anyone have any idea why this happens and/or any usefull solution?
Thanks in advance.
kristo