how to fix vauxhall zafira 2.0 dti 2001?

Have you checked the belt which drives the steering pump is intact and not slipping?

Have you checked there is enough power steering fluid in the reservoir?
If the level has dropped check the pipe connections are correctly tightened, especially the banjo bolts at the steering rack. Also check the rubber gaiters on the ends of the rack, if they are full of fluid, or leaking, it means an internal seal has failed in the rack.
If topping up make sure you use the correct fluid, NOT engine oil or brake fluid.

frank
March 2012