2005 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.

Since this error can result in the ECM operating in a closed loop with the oxygen sensors, the air to fuel ratio for the engine may become excessively rich or excessively lean. At the very best, this will result in additional fuel consumption, but lean conditions are hazardous to engine components due to excessive heat. This should be addressed right away to prevent adverse effects and collateral damage to the catalytic converter, exhaust manifold, and engine cylinder head valves.

The air injection system is typically trouble free, however, when issues do arise from these systems it seems to occur between the 80,000 and 120,000 mile mark. This is no guarantee, and most vehicles will never have an issue with this system due to its lack of complexity and design to operate near high heat.

Diagnosing the control solenoid for the air injection system varies by make and model, but generally the technician will attempt to actuate the solenoid with vacuum or through the engine control module via a diagnostic scan tool. If the solenoid fails to actuate in either direction, the solenoid will be replaced.

Most times the air injection system control solenoid will be mounted at the pump or directly on the control valve between the pump and the manifold. This means replacement may involve removing a portion of the metal line, the control valve, or an individual component located adjacent to the system. In any case, replacing the solenoid is a matter of disconnecting, removing fasteners of some type, and reinstalling the new solenoid. Some manufacturers may require that the solenoid is learned by the ECM before operation of the vehicle.

Since the air injection system uses rubber press-fit hoses and other components subject to deterioration, any time the system is serviced it is advised to perform a complete inspection of the secondary air system. This includes inspecting for exhaust leaks where the air injection system connects to the exhaust manifold or exhaust pipe.

Portions of this system become extremely hot to the touch as they are directly connected the the exhaust system. Any time this system is service, ensure that the engine has had adequate time to cool. Laser thermometers are advised to locate high temperature areas prior to servicing the system.

Replacing the air injection idle solenoid can either be very simple, or very difficult. Many vehicles will actuate the air injection control valve with vacuum, and many others will actuate the air injection control valve via a computerized electronic circuit. This means diagnostics of the valve will differ by vehicle, and may be more difficult or intricate than expected. Vacuum type solenoids are the only type recommended for replacement and diagnostics by the untrained DIY mechanic.