I have two new Bosch sensors arriving today. I need at least one to pass smog here in CA. I passed the sniff test, just not the check engine light.

Here are my codes:
P0158
P0138

I'm replacing the primary/upstream first.

Which one do you think on the downstream I should replace?

 

The one with the lazy voltage cycles on your voltage trend. Don't have that? Then you have no real answer to your question. And, on the X-300, the downstream sensors control the fuel trim, not the upstream as on most cars.

 

I realized there are four. I just ordered two more. Screw it. They are all OE with 116K on them. Out with the old in with the new...MPG!