How to Test Fuel Injectors - Checking the Trigger Circuit for the Injectors

1. Connect a test light to the positive terminal of the battery. Take the same test light that you used for the previous test, but this time connect the alligator clip to the positive terminal on the battery instead of the negative.

• You can identify the positive terminal by looking for the positive sign (+) on the battery or the letters POS.
• Make sure the alligator clip has secure, metal on metal contact or the test light will fail to function.

2. Have a friend start or turnover the engine. Have a friend start the engine. If the vehicle won’t currently run, have your friend attempt to turn it over as you test each injector. Make sure you do not have any clothing or body parts hanging into the engine bay as it starts or turns over.

• If the engine won’t start, remember that attempting to turn it over for too long can kill the battery and damage the starter. Only attempt to turn it over with the test light in place.

3. Probe the opposite wire with the test light. Use the test light and check the opposite wire of the constants you identified in the previous test. Press the sharp end of the probe through the rubber coating firmly until it makes contact with the metal wire inside.

• Be careful not to press the probe all the way through the wire and out the other side.
• Always cover holes in the wire’s protective coating with electric tape once you’re done.

4. Look for flashing or flickering light. With the engine running at an idle, the test light should flicker dimly and as your helper applies throttle by pressing the gas pedal, the light should flicker more brightly. This light represents the signal being transmitted by the ECU to the injector to spray fuel. If the test light is failing to light up, the injector may be bad or there could be an issue with the electronic control unit for the vehicle.

• This issue could be caused by a failing ECU, or one of the injectors along the fuel rail may be faulty.
• The electrical pulse is transmitted through each of the injectors to one another, so one faulty injector could cause issues in multiple injectors.

5. Disconnect the wiring clips to each injector and begin the test again. With none of the injectors connected, the flickering pulse should transmit through all of the wires without any issue. Use the test light to confirm this on the wire for the last car fuel injector clip (at the end of the fuel rail). Keep the test light connected as you reconnect each fuel injector one by one. As you connect each injector, the pulse intensity should remain the same. It shouldn’t change until you connect a faulty injector that creates too much resistance for the pulse to travel through easily.

• When the pulsing light dims as you connect one of the injectors, that injector is faulty and needs to be replaced.
• You can purchase new fuel injectors for your vehicle at most auto parts stores.