I want to replace the O2 sensor on my stock rebuilt A6. I see that Napa has a bosch "universal" for $21, a "OEM type" for $39, and Honda sells them for >$150. Does it matter? I just want to pass CA smog...

 

Doesn't matter. Just make sure it's a 1 wire (which for that price it most definitely is).

 

If I a not getting a CEL, can the O2 sensor still be bad?

 

In my personal experience and that of others on sites like this one, the generics are hit and miss. I tried 2 Bosch generics that only worked marginally well. I ended up buying OEM.

And yes, the 02 sensor can be slow and lazy enough to affect performance but not trigger a CEL because there is at least a little bit of output to keep the ECU happy.

 

Can it be slow and lazy enough to cause a failed NOx on a CA smog? I am trying to pass smog as cheaply as possible....