Fuel injectors operate by voltage pulse. Positive voltage is supplied to the injector as long as the engine is running. The car's computer turns the ground signal on and off to rapidly pulse the injector. The longer the ground signal is on, the more fuel is injected into the engine. A short ground pulse will keep the engine speed low. Using a voltmeter to perform an injector voltage supply test requires an assistant and is an important safety step to prevent major engine damage.
Remove the fuel pump fuse. This will prevent the car from starting while a crank test is being performed. In the event the car will not start, it will also prevent large quantities of gasoline being injected into the engine cylinders. The gas will wash away all the oil that protects the pistons and cylinder walls. As the piston travels up and down during testing, the engine will be ruined.
Unplug the injector in question and insert the positive voltmeter lead into one of the two connector slots coming from the injector wiring harness. Touch the other lead to the engine metal --- not plastic --- for a ground. Have an assistant turn the key on. If there is no voltage, test the second connector slot. There should be battery voltage on one of the two, normally 12.6 volts. If not, the voltage supply circuit needs to be repaired.
Test the injector circuit for ground. If there is voltage present at the connector, insert the ground voltmeter lead into the open slot of the connector. Have an assistant crank the engine over. The voltage reading should fluctuate as the ground signal is applied and then removed as the engine cranks over. If a digital meter is being used, a reading will be difficult to determine as the voltage drops to zero then back. This is because the meter response is too slow. However, a ground signal is present and the circuit is normal. If there is no ground signal, the problem lies with the computer or related wiring.
A quick test to see if the injector is operational is to use a mechanic's stethoscope. Touch it to the injector while the car is running and listen. A ticking will be heard.