Fan not working

Hi guys I'm looking for a bit of advise. I have Mg zr for 8 years now has had all the work you can think of done on it the usual hgf gearboxes clutch rad all been replaced. My problem is the fan won't kick in itself I replaced the temp sensor on the coolant the one on the front of the engine and the fan still won't work. If I unplug it the fan comes on straight away and stays on till I turned engine off so I know the fan is working it's just not getting the message to do so. Anyone able to shed some light that might be the issue as one day came off motorway and stopped in traffic when it suddenly started to overheat. Also the coolant bottle had blown all the coolant over the engine and the coolant bottle was empty.

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