My 2001 V70 has the check engine light on and I pulled code P0422: Main Catalyst efficiency below threshold. From previous posts I understand this can be due to a bad catalytic converter, a bad oxygen sensor or leaky vacuum lines. I haven't found any leaks, and it seems that O2 sensors generally go bad before the cats, and out of the two sensors, the upstream one generally goes bad first.

I used my code scanner to get live reads while driving, but only Sensor 2 showed up. Does anyone know why that would be the case? If Sensor 1 has stopped working completely, I would have gotten another code wouldn't I? (Something about Insufficient Switching or Transition Time Ratio?) And the check engine light wouldn't have gone off and come on again later, it would have been on all the time, right?

Before I replace Sensor 1, I would like to know that this is the problem. Does anyone know anything that can help me?