An oil pressure gauge is the main component that shows the driver what the oil pressure amount is while the engine is running. The oil is pumped out of the oil pan and throughout the engine. Once the oil passes across the oil pressure sending unit, the unit sends the proper pressure amount to the gauge. It's vital that the gauge is working properly at all times. A faulty gauge can cause serious engine problems if not caught in time. The sensor can be checked with a multimeter, but the gauge itself must be checked with a potentiometer.
Look over the vehicle's owner's manual in the oil pressure specifications section and make a note of the proper pressure amount, voltage and ohms.
Open the bonnet to the vehicle and secure it in place. Locate the oil pressure sending unit that's near the engine's oil filter. Grab the plastic clips of the wiring harness plug that hooks up to the sensor and unhook the plug from the sensor.
Crank the engine and let it run at idle speed. Hook the leads of the multimeter to the prongs inside of the wiring harness plug that connects to the sending unit sensor. Adjust the settings on the multimeter to the "Volts" section. Make a note of the readout on the multimeter. The readout should be near or the same as the "Volts" specifications in the owner's manual. If the readouts are wrong, it could mean faulty wiring or a faulty oil pressure gauge.
Turn off the engine and move the settings on the multimeter to the "ohms" setting. Make a note of the readout. The readout should be near or the same as the ohms specifications in the owner's manual. Unhook the leads from the wiring harness plug. If the readings are wrong, it could also mean faulty wiring or a faulty gauge.
Loosen and remove the oil pressure sending unit sensor from the engine with a ratchet and socket. Place the sensor in a safe area. Screw the hose end of the manual oil pressure gauge into the oil pressure sending unit sensor hole. Crank the engine and let it run at idle speed.
Make a note of the oil pressure amount on the manual oil pressure gauge and compare it to the oil pressure specifications in the owner's manual. The oil pressure amount should be near or the same as the specifications inside of the owner's manual. If the manual oil pressure gauge is reading out at the proper specifications but the oil pressure gauge inside of the vehicle isn't, the problem could be the sensor, wires or the oil pressure gauge. To check the dash gauge itself, remove all necessary components to get to the oil pressure gauge.
Hook the red positive lead of the multimeter to the hot wire on the back of the gauge. The hot wire is the only wire that will show voltage. Hold the black lead of the multimeter to one of the other wire connectors on the back of the gauge. Make a note of the multimeter reading. The multimeter reading should be the same or near the oil pressure gauge specifications inside of the manual. If not, the gauge is faulty.
The key is to ensure the readouts from the multimeter are the same or near the volts and ohms specifications inside of the owner's manual. The same volts and ohms specifications for the sending unit sensor, wiring harness and the gauge should all be the same or near the specs inside of the manual. The sending unit sensor and the wiring harness plug from the sending unit must be tested as part of the oil pressure gauge testing process. If you don't have the owner's manual for your vehicle, you can get a new manual at most any dealership for the specific make and model. If not, you can find a maintenance and repair manual for the make and model vehicle at most auto parts stores. The maintenance and repair manual also includes the proper oil specifications, voltage specifications and ohms specifications for the oil sending unit, wiring harness and the oil pressure gauge.
Do not continuously run the vehicle after the oil pressure gauge has stopped working. This can result in damage to the engine by not knowing what the oil pressure is.