Speed sensitive power steering
If the car has speed dependent power steering to control the steering assistance, there is a solenoid which controls a hydraulic valve on the steering gear.
The solenoid is controlled by the power steering control module. This receives speed data from the directly connected speed signal from the central electronic module (CEM).
The power steering control module uses a signal to check the solenoid. The solenoid deploys the valve on the steering gear to the required degree and regulates the oil in the steering gear to reduce or increase the servo assistance. The steering assistance increases at lower speeds and is reduced at higher speeds.
There are no diagnostics for the power steering.