O2 sensor reading

Im a little confused on what the 02 sensors reading pre and post cat are suppose to be at, the pre o2 sensor good when it jumps up and down? and a post cat sensor suppose to read a steady 500-600 reading??

Generally speaking, yes. Conventional O2's in an upstream position will switch. Downstream sensors will USUALLY hang around 600-800mV if the catalyst is healthy and performing correctly.

I say usually because I see more and more vehicles with downstream sensors that switch frequently, or appear to follow the upstream, and still pass their catalyst monitors every time. Basically, don't condemn a catalyst based solely on the downstream sensor. It's not bad 'till the car decides it is, IMO.

Thanks for the reply I appreciate it

And do you know if the readings would change or make any difference if the oxygen sensors would be moved further from the catalytic?

Within reason moving either sensor shouldn't make any difference but it could affect how quickly the sensor reaches working temperature . The criteria for flagging a PO420 is based on "crossing counts" that is how often the post cat sensor crosses through the 500mv line and compares this with the pre-cat sensor. Too many or too few post-cat signal crossing through the 500mv line and the money light is turned on. There have been cheats used to fool the engine computer by physically partly shielding the post-cat sensor from exhaust flow but on recent car the engine computer is more difficult to fool.

LOL! The money light. I have to remember that.

Thanks Andy.Macfadyen, I’m new to the diagnostician field and I’m trying to understand how these oxygen sensors work,

This should explain it a a bit --- this is for conventional Zirconia narrowband O2 as wideband A/F sensors are a bit diffeent. Sorry for the quality I had to screencap the images from a PDF as I lost the original jpegs.

Thanks for the graphs it really helps to picture it better