It mentions about a potentiometer (variable resistor) im asuming the egr has different positions which is determined by a millivolt (mV) or milliampere (mA) siginal if so just say 20mA is produced when valve is fully closed find out the resistance at that point and bridge it with a fixed resistor of that resistance to simulate fully closed all the time! I doubt it will be that easy though because it will probably have to meet other parameters also