How to Tell When Your Fuel Injectors Are Going Bad

Updated February 21, 2017

For an engine to operate at optimal efficiency, the fuel-air mix must contain the right ratio of fuel to air. The power train control module (PCM) takes inputs from engine sensors. Based on what it receives, it sends electrical signals to the fuel injectors to determine how long they stay open and hence how much fuel enters the intake. While fuel injectors don't break very often, they do go bad from time to time. If you suspect a problem, identify and test a broken fuel injector.

Broken fuel injectors can manifest a variety of symptoms. Misfiring, rough idle, hesitation, stumbling on acceleration, loss of power and poor emissions are all common signs of defective fuel injectors. These symptoms, however, can also be caused by other problems, so in order to determine whether your fuel injectors are at fault you need to test the injectors and rule out other possible causes.

Most new-model cars will display a check-engine light and store a trouble code in the event of a problem. If you have any of the symptoms listed above and your car's check-engine light is on, retrieving the trouble code can help you to determine the origin of the problem. Trouble codes may vary by manufacturer and are not universal.

If the trouble code indicates a problem that might be related to the fuel injectors--a misfiring cylinder, for example--or if your car doesn't store trouble codes, you can use a stethoscope to determine whether the injectors are broken or clogged. With the engine running, touch the stethoscope to each injector. If the injector is working properly, you should hear a series of clicks as the valve opens and closes with each cycle.

With the engine off, unplug the electrical connector to each of the fuel injectors. Attach the two voltmeter leads to the two prongs of the fuel injector. The resistance should be within the manufacturer's specifications. If it is not, replace the injector.

To test for leaking fuel injectors, remove the fuel rail and injectors, following the procedures outlined in the manufacturer's instructions. Place a piece of paper towel or shop cloth beneath the fuel injectors and turn the ignition to the ON position, but do NOT start the car. If fuel begins to drip from one of the injectors after a few seconds, the injector in question is leaking.


Fuel systems are under pressure. ALWAYS follow the manufacturer's safety instructions and take appropriate safety precautions when working with the fuel system. Never allow fuel or fuel vapour to come in contact with heat, sparks or cigarette smoke. Never attempt to remove fuel system components without first depressurising the system.

Things You'll Need

  • Voltmeter
  • Stethoscope
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About the Author

Based in San Diego, John Brennan has been writing about science and the environment since 2006. His articles have appeared in "Plenty," "San Diego Reader," "Santa Barbara Independent" and "East Bay Monthly." Brennan holds a Bachelor of Science in biology from the University of California, San Diego.