Fuel injectors need voltage to perform correctly. The noid light determines if your fuel injector receives its proper voltage pulse from your car's computer. You use the noid light to check for voltage pulses at the electrical harnesses for the fuel injector. On some cars, depressing the accelerator when you start the car can cause the voltage signal to the fuel injector to automatically shut off, giving you a false result.
Disconnect wiring harness connector. You need to undo the connector from the fuel injector. On some car models you may find it difficult to locate the fuel injector's electrical connector.
Plug the noid light into the harness connector.
Disable the ignition. After you're sure the ignition won't start, you want to turn over the engine at the starter motor.
Check the noid light for flashing. If it flashes rapidly, your fuel injector is receiving a voltage signal. If you see no flash, you have an open in the power feed or ground circuit in the fuel injector, which could mean that your car's computer or connecting wires are defective.
