How to Tell Which Diesel Injector Is Bad

Updated July 19, 2017

The fuel injector is part of your fuel distribution system. Fuel is drawn from the fuel tank by the lift pump, and then passes through the fuel filter before being sent either to the injection pump or directly to the injectors. The diesel fuel is then sprayed into the cylinder during the intake stroke of the engine. Failed injectors will cause the engine to run below normal operating horsepower.

Raise the hood of the vehicle and start the engine. Let the engine reach normal operating temperature which is between 82.2 and 90.6 degrees Celsius.

Point the thermo-gun at the exhaust manifold. The manifold has exhaust outlets which are indicated by the number of cylinders the engine has. Point the thermo-gun at each outlet of the exhaust manifold. Record the temperatures at each outlet.

Refer to the service manual of the engine you are working on to determine if the temperature range falls within specifications. An exhaust manifold outlet that reads below normal operating temperature may be indicative of a failed injector.

Locate the vehicle's Deutsch connector, which is usually located under the dashboard on the driver's side. The connector will either be rectangular or round in shape. Remove the protective cap from the connector.

Connect the diagnostic reader to the receptacle under the dashboard with the proper connector as required. Refer to the vehicle's service manual to determine which connector is needed.

Perform cylinder cut-out tests using the diagnostic reader. Cylinder cut-outs should be performed in firing order of the engine. Continue to shut-off cylinders until the diagnostic reader finds the cylinder which is not firing. This is indicative of a possible failed injector.


Diagnostic software and readers are needed on all electronically driven engines. Some companies specialise in testing injectors. It may be more cost effective to pull all the injectors out of the engine and have them bench-tested at a fuel-specialist.

Things You'll Need

  • Mechanics tools
  • Diagnostic reader and software per manufacturers requirements
  • Non-contact thermometer gun
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About the Author

Jeff Woodward has been writing since 2007, mostly for "Macabre Cadaver" Magazine, conducting interviews and movie and music reviews. Demand Studios has allowed Woodward to enter the nonfiction article writing market. Woodward's experiences as a parts manager in the trucking industry allow him to write articles for eHow.