NB O2 Sensor Pig Tail Location

Anyone know where the pig tail of the first O2 sensor (the one before the CAT) connecting to the main wiring harness? I tried to follow the wire from the bottom of the car. It disappeared on top of the transmission housing.

My situation is that my 99 has a P0421 CEL. The car had CAL emission and was cconverted to Fed by ditching the first CAT with the use of a RB header. I have a new second O2 sensor (the one behind the CAT) and CAT. Bth less than 6 months old. I suspect it is the first O2 sensor that gives me the code. My car has over 140K miles, therefor it is reasonable that the first O2 sensor is dead. By the way, is there is way to test the O2 sensor.

Also, are those two O2 sensor are identical?

Any info. and suggestion are welcomed!

the one after the cat goes into a plug that runs into the car under the drivers seat i believe. they should be the same except for the wire length. hope this helps!

The one BEFORE the cat plugs in on top between the valve cover and firewall on drivers side. AJ is right about the rear one's location.

Gab, do you still have the instructions for the racing beat header install? A clue to your problem may lie in there considering you had a california emmisions car and an extra cat.

Got it done! Wasn't that bad once I had them taken out and trace them back to the main harness. I picked up some OEM O2 harness connector from thejunk yard and fashion the connections. This is the way to go. From now on, I do not have to splice them in each time I change the O2 sensors anymore! Thanks! Yes, location was shown on the RB instruction. Thanks BAM!