The Mk4 Astra has an electrically driven power pump. Most commen reason for failure is a faulty alternator believe it or not. No power from alt means no power to pump. Check the alt belt hasn't snapped and that's it's charging.
There is no belt fitted to the Mk4 pump to the best of my knowledge, earlier Astras may have had one but not this model,
Ken
October 2006
Power steering comprises of two main parts: The Pump driven by a belt and the steering box. First check that the pump does not have a broken belt which is the most common fault. After that I suggest a Haynes manual.
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