The power steering fluid level is low.

Fill power steering system. Refer to Checking and Adding Power Steering Fluid .

The hydraulic system is faulty.

Diagnose the hydraulic system. Refer to Power Steering System Test .

The power steering pump flow control valve is sticking.

1. Flush the power steering system. Refer to Power Steering System Flushing .
2. Replace the power steering pump flow control valve. Refer to Power Steering Pump Flow Control Valve Replacement - Off Vehicle .

The power steering pump pressure is insufficient.

Replace the power steering pump.

Refer to Power Steering Pump Replacement .