What does it mean?
Fuel atomization is critical to the performance and efficiency of the internal combustion engine. In combination with the heat of the engine and cylinder compression, as well as the swirl effect of the intake air, the modern fuel injector does a pretty good job atomizing and vaporizing fuel for combustion. When the engine is cold, however, fuel tends to remain in its liquid form, which does not burn.
More fuel needs to be injected to compensate for lower vaporization, resulting in less efficient cold engine operation and increased emissions. To improve fuel vaporization, particularly with a cold engine, some automakers have adopted the air-assisted fuel injection system. The system pulls air from the intake, before the throttle plate, and routes it to the fuel injectors via hoses, tubes, or intake manifold channels.
Fuel is injected in the normal manner, using electrical pulses from the ECM (engine control module), into a small chamber. There, vacuum from the intake manifold pulls air into the chamber, swirling with the fuel, leading to better atomization. Then, the air-fuel mixture enters the cylinder, where it is fully vaporized for combustion. Depending on system design, air-assist may be controlled by a dedicated solenoid valve or by the idle speed control valve.
Generally-speaking, the ECM increases air-assist when the engine is cold and is running at idle or part-throttle (low-speed low-load), to maximize the effect of engine vacuum on the system. At higher temperatures and throttle openings, the air-assist system is not active. The ECM (engine control module) uses feedback from the air-fuel ratio (AFR) sensor to monitor the air-assist system.
If the ECM detects that the air-assisted fuel injector system is malfunctioning, either letting too much air into the system or not enough, it sets a DTC (diagnostic trouble code) in system memory, P0066 – Air Assisted Injector Control Circuit malfunction / Circuit Low, and illuminates the malfunction indicator lamp (MIL).
Note: Circuit Low Input codes are often the result of low battery voltages (that can have many possible causes), bad connections across electrical connectors or previously repaired wiring, as well as corrosion in electrical connectors.
Other possible causes of low input voltages include poor installation of aftermarket components, poor quality aftermarket components like fuses, relays, and switches, and modification of the electrical system that could include the use of conductors that are not rated for use in a particular application. However, poor connections often result in high resistances in some parts of the circuit, which is why it is important to perform resistance and continuity checks during the diagnostic procedure.