Where exactly is the O2 sensor, in which part of the exhaust?

I have a ITR in a EG hatch. Is the O2 sensor the one in the bottom of the headers or the one in the CAT?

 

Bump that ****!

 

The sensor in the header is the primary O2 (used by ECU to regulate A/F ratios in closed-loop mode) and the sensor in the cat is the secondary O2 (used by the ECU to determine whether the cat is working properly).

 

thanks for that dude, i tought i never get a reply.

So what you're saying is if i ran open header, i'd get a light on my dash for the CAT but the sensor in the header would still be there so my A/F ratios would be Ok?

So when exactly does ECU go into closed loop mode?