I have a 1994 Dodge Interpid 3.3L and code 21 is come up. The left O2 sensor volts goes up and down like it is supposed to, but the right O2 sensor is negtive volts and does not move. A few weeks ago my cat converter got cloged and melt the EGR valve senosr which is plastic and has vacum lines go to it and a electric conection not the metal part which hooks up to the exhaust. My brother got a used plastic part of the EGR valve from a 4 cinder Straits it look the same as original so we used it. I was thinking since the EGR valve plastic peace was from a 4 cinder could it be only sending voltage to one O2 Sensor the left side and not the right. Replace the right O2 sensor and still has code 21. I am going to try swiching the left O2 sesnor to the right and the right to the left. Need help ASAP.