My radiator fan is running when the ignition is turned on???

No fault plugs are fitted together ie (Green reset plugs)
My fault code indicates coolant temp sensor- 21. its the second sensor i have fitted, although the last time it wouldn't start (intermittently)
Could this be again a fault with the sensor itself,
Thanks Lads

 

Not often a second coolant temp sensor would fail - any melted wiring (from the headers) running across the bottom of the radiator ??