Before & After Cat O2 Sensors
 

What are the symptoms of having a bad/poorly operating before cat O2 sensor VERSUS an after cat O2 sensor? The after cat sensor just tells you if the Cat Convertor is working or not?

Probably nothing you could feel driving the car. Did you bust an emssion test?

The comparison of the O2 level pre and post catalyst are part of OBD II and if any readings are outside the limits you should get a check engine light and appropriate code.

Duke

No. The car actually passed the CA test in January but in April, I got a CE light. I pulled the codes and got a P0411, 2ndary air injection problem incorrect flow. I had this before where the plastic hoses had cracked or been pinched. Checked those, no problems seen, no driveablity problems and reset the code. Came back a couple times over the next 2 weeks (same code). Read all the posts....and gave a shot in the semi-dark and replaced the pre cat O2 sensor, took the gamble but figured it was due for one, but hey, the CE light has not come back on since. Of course, idle isn't any smoother, more power, better mileage, despite what the Bosch marketing team claims. So I was just wondering if the post cat O2 had any correlation with ?????? anything? Probably not, right? Will have to wait for the next problem that shows up...

I can't offer any more insight on the O2 sensors other than what I previously mentioned. The second O2 sensor essentially monitors catalyst efficiency. As to why a new primary O2 sensor would correct an air injection fault, I have no idea.

Duke

<<As to why a new primary O2 sensor would correct an air injection fault, I have no idea.
>>

Quite common.

He had an 02 sensor that would not read lean enough to pass the Air Injection self test . On this test , the DM looks for the 02 to go way lean when the pump comes on, verifying pump activation and system check.
So, an 02 that does not read as well as it used to shows up as an air code way before it shows up as a bad 02 sens code. Which will eventually happen...
The diagnosis was correct.