Audi S4 Air Injection System Control Solenoid Replacement Cost

A control solenoid will either allow or disallow electrical power, air or fluid from moving between connected cavities or wires. In this case, the air injection system control solenoid allows or disallows air from moving between the air injection pump and the exhaust manifold. The purpose is to prevent excess clean air from moving into the exhaust during lean conditions, and allowing clean air to move into the exhaust manifold during rich running conditions.

When this solenoid is acted upon by engine vacuum or computer control, depending on the year make and model of the vehicle, it causes the air injection system control valve to open. This allows pressurized clean air from the air injection pump to pass through the control valve and into the exhaust manifold. When the solenoid is actuated in the other direction or released, the control valve will close, preventing the injection of clean air into the exhaust system. This allows optimum performance of the vehicle emission control system, especially the catalytic converter.

If the control valve for the air injection system fails to open or close, the oxygen sensors will detect a lean condition when the valve is open and a rich condition when the valve is closed. This is because the oxygen sensors are expecting less or more clean air to be injected when the engine control module commands the solenoid open. At the very least, the check engine light will illuminate, and codes will be stored for the air injection system, catalytic converter, oxygen sensors, mass air flow sensor, or manifold absolute pressure sensor. The vehicle may also hesitate during acceleration, misfire, or even stall under some conditions. Rough idling and acceleration are also typical if the valve is fully open at all times.