1998 Audi A4 Quattro Air Injection System Control Valve Replacement Cost

Generally speaking, control valves allow or disallow air or fluid from moving between connected cavities. In this case, it will allow or disallow air from moving between the air injection pump and the exhaust manifold. The purpose it to prevent excess clean air from moving into the exhaust during lean conditions, and allowing air to move into the exhaust during rich conditions.

When the air injection system control solenoid is actuated by vacuum or computer control, the solenoid will act on the air injection system control valve, causing it to open or close. When it opens, it creates a path for clean air from the air injection pump to move through the air injection system line between the air injection pump and exhaust manifold. Once in the exhaust manifold, the air will allow improved operation of the catalytic converter by encouraging complete burning of unused fuel before exhaust gases are passed into the atmosphere. When the valve is closed, the lack of air will prevent lean running conditions to be sensed by the oxygen sensors to maintain engine efficiency.

If the control solenoid fails to open or close the control valve for the air injection system, 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.