Most likely the pump. Try to pull the pulley away from the pump. I've run into two where the shaft broke so the pulley wasn't turning the pump. It broke far enough in that no fluid leaked past the seal either.

You have a "ZF"-style pump. It is common for the vanes to wear but the symptom for that is no power assist at idle, but it returns when you increase engine speed. There can be internal leakage in the rack and pinion assembly too but none of these problems occurs suddenly. They will get gradually worse over time and usually will be less severe when the power steering fluid warms up. If in doubt, the dealership has a test gauge. It uses a valve to block fluid flow. If pressure builds quickly, the pump is okay and internal leakage in the rack is the cause. If no pressure will build up, the pump is weak.

Answered by caradiodoc (expert)
17,324 answers provided