How to Test a Bad Fuel Injector

by Dan Ferrell
itstillruns article image
digital multimeter 3 image by dinostock from

A bad fuel injector in your vehicle can cause your engine to idle roughly, can increase fuel consumption, and can lead to other performance problems. In most cases, you can trace problems with a bad fuel injector to a clogged nozzle, stuck valve or failed coil. With the correct tools, you can diagnose a suspected bad injector before you decide to replace it unnecessarily. This procedure applies to fuel injectors found on electronic fuel injection, or EFI, systems only.

Step 1

Park your car in a well-ventilated area. Open the hood of your vehicle and apply the parking brake.

Step 2

Set the transmission in Park (automatic transmission) or Neutral (manual transmission). Block the rear and front wheels with wooden blocks and start the engine.

Step 3

Put on a mechanic's stethoscope and probe the suspected fuel injector by placing the tip of the tool against the injector's body. You should hear a series of clicks as the injector's valve opens and closes. If you do not hear the clicks, the injector is bad or not receiving power. Turn off the engine.

Step 4

Unplug the electrical connector on the suspected fuel injector. Set your digital ohmmeter to a low range on the Ohm scale. Turn on the meter and probe both electrical contacts on the injector with your meter probes. If you get a blank screen on your meter's display, the coil inside the injector has an open. A reading of "0.00" on the meter's display indicates the injector's coil has a short to ground. In each case, you need to replace the injector. If your meter's display shows a resistance value other than "0," check the service manual for your particular vehicle to make sure you have the correct value and proceed to the next step.

Step 5

Connect a nod light on the harness connector of the suspected fuel injector. Start the engine. The nod light should flash on and off. If you did not hear the injector clicking in step 3, this means the control module is feeding power to the injector, but the injector is not responding and should be replaced. If the nod light does not flash, you need to check the circuit and components that feed power to that injector.

Step 6

Turn off the engine.

Step 7

Plug in the harness connector to the suspected fuel injector and close the hood.

More Articles

article divider