Refrigerant and fan leakage does not work

Hello friends.
I am somewhat desperate with my car for two main problems. The first is about the loss of coolant that has been having for at least one year; The workshop has changed the radiator for a new one and the problem persists, then they told me to put a new lid to the cooling fluid (expansion vessel I think it's called) and the problem continues, so I no longer trust my workshop "of trust". Regarding this problem, what I observe is that in the cooling liquid of the expansion vessel there are floating some "hilitos" of dark fat. At the moment, I solve the problem by adding coolant but I would like to know if the problem you have can degenerate in something worse (to save in case I have to finally take it to a workshop). And this would like to know about the second problem that my car has: the radiator fan is never activated even when the temperature reaches the "prohibited" index, would it be enough to stop the vehicle (and let the engine cool) every time these overheating are presented (to save me the workshop repair) or have to take it to the workshop with emergency so that they repair me? Could I repair myself? Does anyone tell me how?

Greetings to all, in any case, and thank you very much, in advance, for attending to me.

Hello partner, the same thing happens to me in my car, but the fans are activated if I press the A/C button then if the temperature is already low to 90 or even less, but if I remove the A/C button it is heated uploading to the red zone.
The refrigerant fluid also has a brown color, although I do the green.
What I have proven is that it does not cool, although the compressor enters.

Well, let's see if someone tells us what we can do or the most convenient so that the problem does not reach older ones.
Greetings.

The problem that presents is because the tennurable or temperature vulvo sensor is damaged, or a relay, or burn the fan fuse
and if the water consumes you and you do not see where it pulls it it can be because of the reheats that the motor
cub Damage your lucky engine

Thanks for your orientation but I suppose the temperature sensor is the thermostat?.
The theme of the fan fuse does not think it is my case because when giving the A/C button the fans work perfectly. I don't know what is a relay.