Checking the oil pressure of a car using mechanical gauges requires removing the oil pressure sending unit and attaching the gauges, according to Mark Gittelman for YouFixCars.com. Such gauges provide a more accurate reading than the gauge on the car's instrument panel.
The oil pressure sending unit is located on the cast portion of the engine block. Some cars have a light illuminated on the instrument panel, while others have a gauge that receives information from the unit. If the light starts to flicker or the gauge gives erratic readings, Gittelman recommends taking a manual reading. He also suggests consulting the owner's manual to determine the engine speed and acceptable oil pressure reading.
There are several reasons a car might have low oil pressure, according to Ben Wojdyla for Popular Mechanics. If the amount of oil in the engine is low, the oil pump is unable to pick up enough oil to lubricate the engine. The oil pump itself could also require replacement. Wear causes small gaps between sliding parts that allows oil pressure to drop.
Before tearing into an engine with low oil pressure, Wojdyla recommends checking the oil level, and if necessary, replacing the oil pressure sending unit. If the owner does not have access to an oil pressure gauge, replacing the sending unit is another way to rule out problems with it.