Civic EG

I pulled 2 cel codes yesterday and one was 16 which says is injectors and 41 which is primary o2 sensor. My question is where is the primary o2 sensor on a d16z6 and do I only have one o2 sensor ? I changed what I thought was the o2 sensor located on my stock headers yesterday. Also with having a aftermarket exhaust and carb legal intake would that cause the codes to appear. I don't have any check engine light either. My car had been idling odd so I decided to pull the cel codes

If an injector is bad, it will destroy the O2 sensor. It could also set an O2 sensor code.
OBD1 cars have only one O2 sensor.
Aftermarket intake and exhaust wont cause an engine code as long as the O2 sensor is mounted and plugged in correctly.
The codes could have been old. Reset the ECU and see if they come back.