sworkscooper

When I had SAWS Tunning adjust my ECU , they never even opened the boot let alone removed the ECU . It was done through the diagnostic port near the fuses. I was given the option of various stages of tune which would give very mild burbling to ear splitting back fire which I certainly didn't want . In other words you wouldn't see any disturbance around the ECU . I was told the remapping could be reverted to standard if necessary. You would only find out if this is the cause of your back fire if you had the ECU interrogated by a turning company .

Do you know if the car has had the ECU remapped as this will cause this to happen.

I am not too familiar with the F set up . I believe it is similar to the MPI Mini in that if the engine ECU does not see a viable signal from the sensor it will turn the fan on as a safety feature. It could be a poor connection to or from the ECU or as you say just a case of the switch being faulty. Forum members who own an F will be able to guide you better than myself.
John .

I presume as your profile picture shows an F , the sensor is under the boot on the R/H/S òf the engine bay apperture , near the boot hinge , which is the engine bay temperature sensor .