I have a 2008 focus st and I have noticed that my fan is on all the time whilst the engine is running my temperature gauge is showing normal. I have checked all the fuses and they are all ok, I can only assume that there must be a sensor some where on the car has anybody else come across this? or know where the senor is located?

It will either be the engine temp sensor (the ECU will set fans to constant as a failsafe if no valid signal is being received from the sensor), or if you have the heater set to windscreen I believe it automatically turns on the AC to aid demisting in which case it will be the AC fans you can hear.

has it just started to play up, have you checked it with aircon switched off and heater fan off?

I have had a dianogstics test done today and it came back with fault code po111, now search time on google

Intake Air Temperature (IAT) Sensor fault. Could be your problem, that and the Coolant Temperature Sensor (CTS) work together and should have the same readings.

You don't say if it is petrol or diesel but I had that happen when the DPF was clogged and the car was trying to regen all the time. Then I noticed it again when the coolant level was down below the min mark on the tank so maybe there is a sensor which reacts to insufficient coolant as a fail safe? Anyway, ripping out the DPF solved the first point, topping up the collant seems to have solved the second one.