The DISA valve (also known as the air adjustment unit) controls the path of air through the plenum, regulating the intake manifold pressure to provide greater torque around town and more power at higher revs. Unfortunately, these are weak point on these BMW engines, causing the flap to get stuck and potentially allowing debris to go unfiltered into the engine. Located next to the intake manifold, replacement is a relatively simple procedure and should be replaced as part of proper preventative maintenance. These replacement DISA valve units are high quality and a fraction of the cost of a BMW Genuine replacement.