Your car’s ABS hydraulic unit receives commands from the ABS controller. This in turn obtains speed-related data from sensor rings on each wheel. When the sensor ring detects that one of your wheels is spinning at a different speed than the others, the controller tells the ABS hydraulic unit to close the solenoid valve. This then reduces the braking force on the offending wheel and stops it from locking for more than a nanosecond. A faulty ABS hydraulic unit could cause your car to fail its MOT. If your wheels are locking and you see a warning light appear on the dashboard, it could be that your ABS hydraulic unit is not working as it should. To help prevent such issues, we advise buying a high quality replacement from Eurocarparts.com.