How to check a fuel injector

Written by dan ferrell
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How to check a fuel injector
Most automobile fuel injectors in use today are solenoid operated. (Photo courtesy of GodFather1921 at

Most automobile fuel injectors in use today are solenoid operated, requiring a voltage supply to work. The most common problems you will encounter will be clogged or malfunctioning injectors. Follow these steps to test injectors used on throttle body injection (TBI) and electronic fuel injection (EFI) systems.

Skill level:
Moderately Easy

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Things you need

  • Flathead screwdriver
  • Phillips screwdriver
  • Ratchet and socket set
  • Timing light
  • Mechanic's stethoscope
  • Ohmmeter
  • Noid light

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    TBI Injectors

  1. 1

    Remove the air cleaner assembly from the top of the engine. Depending on vehicle model, you can do this by hand, with a flathead screwdriver, Phillips screwdriver, wrench or ratchet and socket.

  2. 2

    Start the engine and let it idle. Connect a strobe or timing light according to manufacturer instructions, aim the light at the throttle body and see the fuel pattern spray. The spray should be an inverted, even V, pattern. If not, have the injector cleaned.

  3. 3

    Have someone else crank the engine if it does not start. You can quickly verify if fuel is being injected into the throttle body just by watching the injector as the engine is cranked. If fuel is being injected, use the timing light as described in Step 2 and verify if fuel is being delivered correctly. Have the injector cleaned if necessary.

  4. 4

    Consult your vehicle service manual to check for power supply to the injector if there is no fuel injected into the throttle body. Each manufacturer provides its own method for this procedure to avoid damage to sensitive electrical components. Alternatively, take the vehicle to an auto repair shop for further testing.

    EFI Injectors

  1. 1

    Start the engine and verify the injector operation. If the car does not start, have someone crank the engine. Use the tip of a mechanic's stethoscope to check the injector's operation. Also, you can place the tip of a long screwdriver against the injector and your ear against the screwdriver's handle. You should be able to hear the injector clicking, meaning the injector's valve is opening and closing. If the injector makes no sound, it is not working.

  2. 2

    Unplug the injector's electrical connector and check the unit's coil resistance with an ohmmeter. Set the meter to a low resistance range and touch each electrical terminal on the injector with the ohmmeter's leads. If you read infinite resistance--no resistance at all--the injector's coil is opened. If you read zero resistance, the coil might be shorted to ground. In either case, the injector should be replaced. If you read resistance across the coil, go to the next step.

  3. 3

    Connect a noid light at the injector harness connector--see the Tip section for more information. Start the engine and watch the noid light. If the noid light flashes, the injector is receiving power, and there might be a failed component inside the injector. If the noid light does not flash, the wiring harness or control module is not supplying voltage to the injector. Take your car to an auto shop for further troubleshooting.

Tips and warnings

  • You can buy an inexpensive injector noid test light for your particular vehicle at most auto parts stores.
  • When working on the fuel system of your vehicle, do not allow smoking near or around the car, and park away from appliances with open flames like water heaters and dryers to avoid a possible accident.

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