Oxygen sensor

Does the downstream (after cat) o2 sensor affect engine running atall or does it just monitor cat operation and inform ecu (for dtc) to set a cat code?

That'd depend on the specific year/make/model that we're talking about. But, generally, the downstream sensor has no input on fuel control.

On the systems that DO have downstream fuel control, the amount of weight the downstream sensor carries will vary. Again, Y/M/M specific.

Istr Mazda have done downstream O2 sensor partial control on some models. The only reason I can think of for using it is to trim the LTF more accurately/faster for different conditions.

So if a cat is removed it can alter the running of the engine? Pain for the boy racers.

Fox.b wrote: So if a cat is removed it can alter the running of the engine? Pain for the boy racers.

As Tyler said, only certain makes/model. In general, the downstream Oxygen sensors have NO control over fuel, so removing the CAT will have no effect on performance. The issue will be that, since there is no catalyst, the downstream Oxygen Sensors will respond exactly like the upstream sensor. This will trigger a P0420/P0430. I believe, this code can be tuned out, if you have tuning capabilities. I have heard of people installing a spacer to lift the downstream O2 sensor out of the direct exhaust flow, slowing down the response time to avoid setting the P0420.

A google search found this picture.

Thanks for the reply, guys.
I'm not a boy racer, I just work in a garage and im trying to get a better idea of how all the systems work. Finding Scannerdanner (and this forum) was a godsend!