OBD2 Sensors >> Fuel Injectors

Fuel Injectors

SPECIAL NOTES: Note that the information provided in this guide is of a generic nature, and is intended for informational purposes only. However, since the basic operating principles of any given engine sensor is largely similar across all makes and models, it is possible to apply the information provided here to a large range of applications. Nonetheless, be aware that neither similarities in operation, appearance, or location, nor effects on engine operation when any given sensor fails is guaranteed, and it is therefore recommended that the relevant technical manual be consulted for details on the location, manufacturer specific diagnostic information, replacement procedures, and other technical information pertaining to the affected application. END OF SPECIAL NOTES.

What does a fuel injector do?

The primary function of fuel injectors on both gasoline and diesel engines is to vaporize pressurized fuel when it passes through the injector, into the engine or combustion chamber. The degree of vaporization depends on the engine, the type and temperature of the fuel used, and the actual fuel pressure at any given moment.

Why is fuel injectors needed?

Modern emissions regulations have forced engine designers to improve the efficiency of the combustion process on both gasoline and diesel applications, and vaporizing the pressure fuel into as fine a mist as possible has gone a long way towards increasing the efficiency of the combustion process.

Essentially, the problem of combustion efficiency involves the propagation of the detonation flame, also known as the flame front. In practice, the air/fuel mixture is ignited only around the tip of the spark plug, and the detonation flame propagates outward from this point. Therefore, by reducing the size of the fuel droplets, mixing of the fuel with the intake air is improved, which in turn, improves the way the detonation flame propagates.

Note that on diesel engines, the propagation of the detention flame does not depend on a spark. On these engines, the combustion process is initiated at several points throughout the combustion chamber, but how well (or otherwise) the combustion process works depends on the size of the fuel droplets, and how well the fuel droplets are distributed throughout the air/fuel mixture.

How does a fuel injector work?

Contrary to popular belief, fuel injectors do not “inject” fuel into the engine. As a practical matter, fuel injectors are merely valves in the pressurized fuel system that are opened by fuel pressure (on some older diesel applications) or by an electronically controlled solenoid to allow a metered amount of fuel to enter the engine.

In practice, fuel injectors form the termination of the fuel system at each cylinder. The fuel pump pressurizes the fuel system, and the injector’s internal solenoid opens and closes a pintle that controls the amount of fuel that is ultimately injected into the engine while the fuel pump and dedicated fuel pressure regulators maintain the overall pressure in the fuel system.

NOTE: While injectors with solenoids and metal pintles have served the auto industry well for many decades, this design is increasingly being replaced by injectors that use stacks of Piezo-crystals/wafers as a control mechanism. Piezo crystals can expand and contract in reaction to electrical impulses much faster than a conventional solenoid can, which makes it possible to control separate injection events much more precisely than is possible to do with other types of injectors.

However, it should be noted that the PCM (Powertrain Control Module) has the ability to control, or adjust the amount of time the injector remains open, depending on the fuel demands of the engine at any given moment. This “ON” or “OPEN” time is known as the injector pulse width, and increasing or decreasing the pulse width either increases or decreases the volume of fuel that is ultimately injected into the engine during an injection event.

While most injection events typically complete in about 2 milliseconds or less, it has become common for most fuel management systems to split a single injection event into several (up to six or more) separate injection events. Injecting varying amounts of fuel into the engine at different times during an injection event greatly improves the mixing of the fuel and air, which improves combustion, which in turn, increases the engine’s volumetric efficiency while using less fuel at the same time.

It should be noted though the effects/benefits of multiple injection events are increased with additional improvements such as specially designed pistons and combustion chambers, increasing the velocity and turbulence of the intake air through the use of intake runner control flaps, forced induction, variable valve/camshaft timing, and sometimes, the use of multiple ignition sparks on gasoline engines.

Where is the fuel injectors located on the engine?

The image above shows the typical location of fuel injectors relative to the inlet manifold and cylinder head on a VW application. Note that while fuel injectors are generally located in similar locations on almost all engines, the actual locations can vary between in-line and V-type engines.

Note however that on some diesel Land/Range Rover applications, the injectors are actually located inside the engine, and removal of the valve cover is required to access the injectors on these applications. Always refer to the manual for the affected application to locate fuel injectors and other fuel system related components.

What does a fuel injector look like?

The image above shows a set of typical fuel injectors. Note that while some design features may vary between different brands of fuel injectors, all modern fuel injectors have this same general appearance.

Possible symptoms of bad fuel injectors

While modern fuel injectors are designed to have reasonably long service lives, they can and do fail for several reasons. Some common symptoms of failed or damaged fuel injectors could include the following-

Decreased fuel economy

This usually occurs on high-mileage engines when the injector pintles have become worn due to long use, or because dirty fuel has abraded or damaged the pintle seats. One other cause is excessive fuel pressure; both conditions cause the injectors to leak fuel during the “OFF” or “CLOSED” cycle, and the only reliable remedy is to resolve the fuel pressure problem or replacement of the injectors if the fuel pressure is known to be normal.

Misfires on one or more cylinders

Injectors that have become clogged over time or as the result of using poor quality fuel can cause misfires. While ultrasonic cleaning of the injectors may resolve the issue in some cases, the better option is to replace the affected injectors. Note that “injector cleaning” additives are mostly snake oil, and their use should be avoided.

The causes of the two symptoms listed above have several other effects, which could include the following-

  • CHECK ENGINE warning light may be illuminated
  • Hard, or no-start conditions
  • Repe3ated spark plug fouling
  • Idling may be rough, erratic, or the engine may not idle at all
  • Varying degrees of power loss may be present at some engine speeds/loads
  • Power delivery may be uneven, or unpredictable
  • Engine may not reach peak revolutions
  • Vehicle may fail mandatory emissions tests
  • Vehicle may emit black smoke from the tail pipe
  • Leaking injectors may cause premature ignition, which in turn, can cause oil dilution and catastrophic engine failure
  • Leaking injectors may cause catalytic converter failure
  • Severely leaking injectors may cause hydro-locking, which can prevent the engine from cranking, which in turn, can cause severe engine damage


It should be noted that since bad fuel injectors could mimic the symptoms and effects of many ignition related issues, including the symptoms of bad ignition coils. Therefore, careful attention must be paid to any fault codes that are present, since bad fuel injectors will not cause ignition related codes to be set.

Bad fuel injectors will be indicated by the generic trouble code listed below, but be aware that defective fuel injectors almost invariably cause, or contribute to the setting of a wide range of emissions, misfire, and/or air/fuel metering codes as well. If such codes are present, be sure to resolve all codes in the order in which they were set and stored to avoid a misdiagnosis.

Below are some common generic fault codes that indicate issues with either fuel injectors, and/or their control circuits-

  • P0261 –  Cylinder 1 Injector Circuit Low
  • P0262 –  Cylinder 1 Injector Circuit High
  • P0263 –  Cylinder 1 Contribution/Balance Fault
  • P0264 –  Cylinder 2 Injector Circuit Low
  • P0265 –  Cylinder 2 Injector Circuit High
  • P0266 –  Cylinder 2 Contribution/Balance Fault
  • P0267 –  Cylinder 3 Injector Circuit Low
  • P0268 –  Cylinder 3 Injector Circuit High
  • P0269 –  Cylinder 3 Contribution/Balance Fault
  • P0270 –  Cylinder 4 Injector Circuit Low
  • P0271 –  Cylinder 4 Injector Circuit High
  • P0272 –  Cylinder 4 Contribution/Balance Fault
  • P0273 –  Cylinder 5 Injector Circuit Low
  • P0274 –  Cylinder 5 Injector Circuit High
  • P0275 –  Cylinder 5 Contribution/Balance Fault
  • P0276 –  Cylinder 6 Injector Circuit Low
  • P0277 –  Cylinder 6 Injector Circuit High
  • P0278 –  Cylinder 6 Contribution/Balance Fault
  • P0279 –  Cylinder 7 Injector Circuit Low
  • P0280 –  Cylinder 7 Injector Circuit High
  • P0281 –  Cylinder 7 Contribution/Balance Fault
  • P0282 –  Cylinder 8 Injector Circuit Low
  • P0283 –  Cylinder 8 Injector Circuit High
  • P0284 –  Cylinder 8 Contribution/Balance Fault
  • P0285 –  Cylinder 9 Injector Circuit Low
  • P0286 –  Cylinder 9 Injector Circuit High
  • P0287 –  Cylinder 9 Contribution/Balance Fault
  • P0288 –  Cylinder 10 Injector Circuit Low
  • P0289 –  Cylinder 10 Injector Circuit High
  • P0290 –  Cylinder 10 Contribution/Balance Fault
  • P0291 –  Cylinder 11 Injector Circuit Low
  • P0292 –  Cylinder 11 Injector Circuit High
  • P0293 –  Cylinder 11 Contribution/Balance Fault
  • P0294 –  Cylinder 12 Injector Circuit Low
  • P0295 –  Cylinder 12 Injector Circuit High
  • P0296 –  Cylinder 12 Contribution/Balance Fault

How to test fuel injectors

Since fuel injectors are not repairable, the average non-professional mechanic is limited to only a few basic tests that may or may not resolve the issue, these tests being-

Testing the fuel pressure

If any fuel pressure related codes are present, follow the manufacturer’s prescribed procedure to test the fuel pressure with a dedicated fuel pressure gauge, and make repairs or replace components strictly in accordance with the manufacturer’s prescribed procedures.

WARNING: Since this test involves making connections to the fuel system, be sure to observe ALL prescribed safety precautions to avoid both personal injury, and the possibility of starting an engine fire that could destroy the vehicle and other property. If you have ANY doubts about your ability to perform this test, refer the vehicle to the dealer or other competent repair facility for professional assistance.

Testing the wiring

If no fuel pressure related codes are present, perform resistance, and continuity tests on the wiring of the affected injector(s) between the injector and the PCM. Replace or repair wiring as required to ensure that all electrical values fall within the manufacturer’s specifications

If the wiring checks out OK, test the affected injector(s)’ internal resistance across the terminals in the injectors’ connector, and replace the affected injector(s) if the obtained resistance readings do not comply with the manufacturer’s specified value(s). Note that there is no single resistance value that applies to all injectors, so be sure to use reference data that applies to the affected application.

Performing a volume test

This test involves testing the volume of fuel each injector delivers during a fixed period of time, and while it is possible to perform this test on a DIY basis, doing so requires the removal of the injectors and the use of a scan tool that can activate the fuel system while the engine is not running.

Since the object of this test is to measure the rate at which each injector delivers fuel in a fixed period, this test can diagnose leaking and/or clogged injectors. However, since this test requires a reasonable degree of technical ability, it is not recommended that persons without the required skills, knowledge, equipment, and tools attempt it due to the high likelihood that some components may be damaged during the injector removal process, or that pressurized fuel may be cause personal injury, or an engine fire.

WARNING: If you have ANY doubts about your ability to perform this test, refer the vehicle to the dealer or other competent repair facility for professional assistance.

How to replace fuel injectors

WARNING: In some cases, it may be necessary to remove or partially disassemble one or more unrelated components to gain access to the fuel rail and injectors for the purposes of testing and replacement of injectors. If you have ANY doubts about your ability to perform the required disassembly or removal of components, refer the vehicle to the dealer or other competent repair facility for professional assistance.

Nonetheless, fuel injector replacements generally follow the same pattern on all applications. Below is a generic procedure to replace all the fuel injectors as a set-

  • Ensure that the engine is cold to prevent burns or scalds, and to prevent the possibility of starting an engine fire. Note that the battery should only be disconnected as a safety measure if the manual explicitly states that the battery can be disconnected without the PCM suffering the loss of vital programming
  • Remove all cosmetic engine covers as per the instructions in the manual
  • Refer to the manual to locate and identify the fuel rail and injectors
  • If removal or disassembly of unrelated parts and/or components is required, be sure to follow the instructions in the manual ERXACTLY to avoid damage to parts and components
  • DO NOT disconnect any fuel lines or any other fuel system related components before releasing the residual pressure in the fuel system, since failure to do this could cause serious personal injury, and/or an engine fire. Note that this step usually requires that you follow a set procedure, so be sure to follow all prescribed steps exactly
  • Once the residual fuel pressure is released, remove the electrical connectors and all spring clips or other retaining devices that lock the injectors to the fuel rail. Refer to the manual for details on how to remove these clips without damaging either the injector or the fuel rail
  • Remove all screws/bolts that hold the fuel rail down, and pull the rail off of the injectors, but note that even though the injector’s retaining clips may have been removed, some injectors may be pulled out of the engine when the fuel rail is removed
  • With the fuel rail out of the way, simply pull the injectors out of their sockets
  • Make sure that the bores of the sockets are clean, and free of grit, corrosion, or general gunk; clean as required
  • Replace all O-ring seals on the new injectors if the replacements did not come with new O-rings
  • Lubricate all O-ring seals with a TINY amount of rubber grease, and insert the new injectors into the previously cleaned sockets
  • From this point onwards, complete the process in the EXACT reverse order of removal, and make sure all retaining clips, bolts, and fuel lines are tightened properly to avoid fuel leaks
  • Start the engine to verify that there are no fuel leaks, clear all codes, and test drive the vehicle to verify that the injector replacement was successful

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