i'll replace my catalyst soon ... i just need to know if the oxygen sensor is placed on top of the catalyst or on the pipe ?? i cant see the sensor if am under the vehicle ..

 

Typically it is in the pipe. To provide more exact information, we need to know what it is you are driving (year & engine at a minimum).

 

got a 98 LT blazer 4wd ... 4.3l v6 Vortec

 

You will have one behind the catalytic converter in the pipe that monitors the catalytic converter. The other two will be between the 'Y' in the y-pipe and each exhaust manifold.

Please put your vehicle information in your signature and attach it to your posts when you ask technical questions. It is necessary for us to be able to answer you correctly.