An oil sending unit, also called an oil pressure sensor or switch, controls the oil indicator light or gauge in a vehicle. The indicator tells the driver of any problems with the oil pressure. Low oil pressure can lead to engine damage, but you can test the sending unit if you believe it isn't accurate. You can buy or rent an oil pressure testing gauge at most auto parts stores.
Check your oil level and condition and add oil if necessary. Improper oil level can have an adverse effect on pressure readings. After adding any oil, turn the engine on again to verify whether an oil pressure problem still exists. Check the indicator light or gauge.
Find the oil sending unit, which will be screwed into the engine block near the oil filter, with a single wire attached to it. Turn the vehicle's ignition switch to the "on" position, with the engine "off."
Unclasp the connector from the unit gently, using a pocket screwdriver if necessary. Take care not to damage the clip that seats it in place, which will likely be covered with a dust boot. Note any change to the dash gauge or oil indicator light. If the vehicle is equipped with a gauge, the reading should go all the way up when disconnected. If it uses a light, attach a jumper wire to the connector terminal and clip it to a good ground on the block or to the negative battery terminal. The light should come on. If not, the problem lies elsewhere in the circuit, rather than in the sending unit.
Turn off the vehicle's ignition and remove the sending unit with a socket, by turning counterclockwise. It is possible to lose a small amount of oil while doing this. Simply wipe the area clean.
Screw in the manual testing gauge firmly, taking care not to strip the threads on the engine block. Route the rest of the gauge and hose into the driver's side of the cabin, away from any moving parts, and tape it to the inside of the windshield with electrical tape, so that it can be read while on a test drive.
Turn on the vehicle and drive it around the block a couple times, while observing the tester gauge. A normal range is typically from 18.1 to 27.2 Kilogram per square inch (psi), depending on the rate of acceleration. If readings are within this range and the previous test didn't indicate a problem elsewhere in the circuit, replace the oil sending unit.
Don't run the engine without the sending unit or the testing gauge screwed into the threaded hole in the block.