Diagnosing fuel injection problems

Troubleshooting fuel injection systems requires logical step-by-step test procedures. Some of the most straight forward ones are supplied by the factory. These tree charts are designed and implemented to save the companies money on warranty repairs and stop the replacement of good components.

port fuel injector Picture

port fuel injector

These car-makers put together easy to follow diagnostic procedures in their factory auto repair manuals that walk the technician through a logical order of diagnosis known as a ladder diagram. You can get your hands on the very same diagnostic tree charts and I have more information about them on my pages dedicated to factory service manuals.

But this post will concentrate on why you should seek out this diagnostic information before you tackle problems you believe are associated with fuel injection.


Taking a hit or miss approach to diagnosing fuel injection problems can quickly become frustrating for even the most experienced mechanics not to mention time consuming and costly.  Most fuel injection systems are tightly integrated into the engine control systems.

The self diagnostic modes of these systems are designed to help in diagnosis when problems are experienced. Unfortunately when a problem does surface many driveway mechanics feel as if the problem will most likely be traced back to the car’s computer. But in my opinion the vast majority of fuel injection problems are found in places other than the computer.

Check the basic systems first

Before you start replacing or ordering sensors remember that poorly operating or weak engine components can often affect the sensor readings. As an example if you have worn piston rings or slack has developed in the timing chain this can affect the air fuel mix resulting in unbalanced exhaust gases that could possibly set an oxygen sensor code.

upstream oxygen sensor Picture

upstream oxygen sensor

If you pulled this oxygen sensor code and replace the sensor without repairing the piston rings or the slack in the timing chain than this code will reset. So before you run out and get an oxygen sensor that may not be returnable take a few minutes to check the more basic items.

These items can be checked even before you purchase a factory repair diagram that contains the diagnostic tree chart that will help keep you on track. Some of the things to check would be that the battery is in good condition and fully charged with clean terminals and connections with no corrosion build up on system grounds.


You also want to check that all of the fuses and fusible links are in good condition and not blown. Perform a visual inspection of the engine compartment area and the wiring harnesses to make sure that they are properly routed and not damaged by unwelcome visitors such as mice and rats.

Checking the fluid levels such as the engine oil level and the level and condition of the coolant is also a basic check that only takes a few minutes if that. When fuel injection problems are suspected it is also a good idea to check that the gasoline in the tank is in good shape and has not been substantially diluted with alcohol or water. I wrote an article on my other blog that provides information on how to test fuel quality.

In the end it is possible that there is a problem with the computer and its network of sensors. In my opinion it is probably slightly less likely than uncovering problems with basic systems such as dirty air filters, clogged fuel filters or worn spark plugs. But nevertheless it is still possible. I had an automotive instructor that used to say the best way to approach problems on modern vehicles is to treat it as if it didn’t have any computers or sensors.

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