There is a high internal leakage.

Refer to Power Steering Gear and Pump Leaks

The power steering gear valve spool is ticking/damaged.

Refer to the appropriate procedure: Power Steering System Test Procedure , or Steering Gear Replacement

The power steering pump pressure is insufficient.

Refer to Power Steering System Test Procedure

The power steering fluid level is low.

Add fluid to the power steering system.

Refer to Checking and Adding Power Steering Fluid .