nissan micra overheating/fan not kicking in?

Hi I have a nissan micra r reg which has started overheating. the radiator is full as is the reserve tank. The fan does not kick in on its own although it does work direct from the battery. I have changed the sensor attached to the cylinder head. I have removed the thermostat and yet nothing seems to be making the fan work properly. Also I have noticed that even when running the fan direct from the battery when I switch the engine on the car begins to overheat again while the fan is running can anyone help please.