An annulus pressure responsive downhole tool (50) includes a selectively actuable deactivating system (66) to render the tool unresponsive to annulus pressure changes. The tool includes a power piston (54), one side (56) of which is exposed to annulus pressure and the other side (58) of which is communicated with a reference pressure which, when it is different from the annulus pressure, results in a pressure differential across the power piston to cause it to move. Connected into the reference pressure side (58) is the deactivating system (66) which upon actuation ensures that the reference pressure is equal to the annulus pressure, thus preventing any pressure differential across the power piston (54).
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