A car's oil pressure sending unit detects the amount of oil pressure in a car's engine and sends that information to the car's instrument panel. It allows the driver to monitor oil pressure and receive a warning of dangerous pressure levels.
Most commonly, the sending unit transmits an electrical output that is interpreted by the car's computer. This action allows the driver to monitor the actual oil pressure constantly via the display on the instrument panel.
Some sending units use a switch that activates when pressure falls below a certain level. Older sending units accomplish the task with a tube connected between the oil pump and a gauge on the instrument panel.