Steering Locking-Up

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Hello fellow S Max owners, this my first post and am really hoping to get some advice after receiving three differing diagnoses from three different garages.

On occasions the steering becomes very stiff only when turning the wheel to the right from the central position. When the wheel becomes un stuck i can here and feel a slight clunk as if something is releasing.

Again, this happens occasionally and does not appear to coinside with weather conditions or starting from cold etc.

I have been assured by one garage that it isn't a problem associated with any mechanical joint beneath the car.The same garage told me it is possibly the steering rack itself,another garage told me it was possibly the pump.

The power steering fluid is also at the correct level and there is no excessive noise made when the steering becomes stuck or when it releases.

Thanks very much in anticipation of a reply !