Hi. I am changing my cats on my 2004 G35x... 148K miles. Would I be wise to change all four sensors while the work is being done? Or is there a way to test the O2 sensors to see if they are all ok? Also, can you buy used oem O2 sensors on this site? Thanks.

 

There are ways to test an O2 sensor, but the whole process is kind of complicated. I'll let you decide if that's something you can handle. O2 Sensor Discussion

You seriously don't want to mess around with used O2 sensors - you have no idea of the history, etc. If you're going to replace them, get new. Otherwise ride out what you have.

 

you can but you need an advanced scan tool that can plot the data.