My car is almost new ( 6000 Km) . 2 weeks ago suddenly there was a steady stop signal and max water sign with fan running at max speed. They told that the water sensor is dead and it was changed. The problem fixed but from 2 days ago l have the same problem but this time in short and long intervals and sometimes after some kilometers there is no sign!!!

''THEY'' had ur car right in front of them, & fixed ur water temp sensor. Good.
l would return to them & explain the same problem. THEY must have checked your:
1. coolant level
2. waterpump working (car is new)
3. thermostat opening normally (new car?)
4. Water is circulating, no radiator problems.
5. The fan comes on when a water temp sensor/ switch IN THE RADIATOR ITSELF is activated. Is this the switch they replaced?

Try starting ur car COLD in the driveway with the radiator cap OFF. Wait 5 minutes watching the car idle, & the water should start to flow past the fill neck. This means ur thermostat is open, & ur water pump is working.
replace cap tightly, & check ur dash lights now. ??

good luck.