It sounds like your sensor is broken, it is mounted on the rear side of the front bumper grille so will need to take bumper off. When you do it make sure you check wiring to it is not damaged as this will cause the fault too. I run cars in mini challenge and this is what happens when you get damage to the front and the wires are cut through.
its behind the black honey combe lower grill on the front bumper
This is the most unprofessional bit of site management I have seen since I was banned from a certain other site...at least they had the excuse that they were enthusiasts who had strong views and not professionals paid to keep thier cool!
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