Air Flow Sensor
WARNING: This page is about a different car, the 1989 BMW 325is, 1989 BMW 325iX, and 1989 BMW 325i. However, it is still accessible from the selected car via links, so may be relevant.
The sensor's mechanical operation depends upon the free movement of the flap inside the main air passage. To check the flap movement, loosen the clamps, remove the rubber intake air duct, and move the flap through its range of travel.
If the flap binds at any point, remove the intake air flow sensor and check for any foreign material that may be interfering with the flaps movement. Check that the flap returns to the closed position smoothly. If the flap cannot be made to move freely, or if the flap's spring tension is uneven, the air flow sensor is faulty and should be replaced.