It's the Pressure sensor that messure the flow through the EGR valve. It messure the differential pressure across an orifice, and compares that to the tables in the ECU to determine whether the EGR is functioning correctly. It's quite intricate and the diaphragm is made from a type of rubber/yrethane, which after extended exposure to the corrosive exhaust gases, detriorates and begins to output false signals.
