OBD0 engine have 2 O2 sensors. one on exhaust #1 & #4 and the other one on #2 & #3, both before the collector. The ecu also have 2 pin for o2 sensors the primary os pin C16 (white cable) and the secondary pin C8 (red/blue cable). My questions is which O2 sensors goes into the primary pin (C16 pin), the #1/#4 or #2/#3? thanks.