98 328I spitting up a code
 

ok I have a p0420 Catalyst System Efficiency Below Threshold (Bank 1) code problem.
so heres what I do know.
98 328I 138k miles
auto enginuity code reader
p0420 is the only code I get.
car runs perfect.

what voltage should the post o2 sensor be at ? and what should the output wave form be? looks like the front o2 and the aft o2 are reading about the same.
I dont want to just start replacing parts until its fixed, I want to diagnose the problem and fix it the right way.


I know that water in the exhaust can kill the o2 sensor and it has been real wet around here.
so I need know to know what voltage ranges I'm looking for and what the wave form should look like..

maybe someone with a good post cat o2 sensor can get a screen capture of the proper wave form and voltage, please...

thanks