just a question ...why is the O2 sensor placed before the catalytic con.

To get "raw data".

 

If it were after the catalyst, the results would be affected by the redox products of the catalyst(s). What you want for A/F ratio control is what comes out of the ports. Granted, the difference would be minor, but there you are.

Not really northwet..

 

The reason the o2 sensor is before the catalytic converter is to maximize catalytic converter efficiency by detecting how much oxygen is before the cat so that the computer can maintain a 14.7:1 stoichiometric ratio to maximize catalytic converter efficiency..

 

There are sensors after the catalytic converter to detect converter efficiency..

after the cat, the o2 sensor would be worthless to the ecu for tuning the air/ fuel ratio, the only reason they put sensors behind the cats on newer cars is for emissions control, basically to make sure that the cats are working, not to control the engine.

the only reason they put sensors behind the cats on newer cars is for emissions control, basically to make sure that the cats are working, not to control the engine.

this is correct.