You need to have the codes read in order to know exactly which O2 is faulty, just as Dave mentioned. However, I could tell you which one you purchased just by the length of the wire. The main O2 has a very short wire about 20cm. The secondary O2 has a very long wire probably double in length or more.
You better hope that the main is faulty because the secondary is a pain in the *** to change.

Vinamg