replace rear o2 sensor or read the following below
from a previous post, not mine)
I have been chasing a P0422 on my 2000 XC70 (140K miles) for a few months.
I may have finally resolved it in the past week.
It was an obtuse, but inexpensive problem to fix.
My approach was to eliminate all the places where un-metered air could get into the intake manifold.... clean and clamp the vacuum hoses; etc.
After checking and rechecking everything, and making some significant improvements in the performance and economy along the way, I still got an occasional P0422 under specific driving conditions.
One more check last week and I discovered that one of the two plastic clips on the intake air temperature sensor was broken off.
I bought a new one from volvo for about $20. It took 2 minutes to replace.
With only one plastic clip, the sensor will work fine at highway speed and a good vacuum in the intake. At suburban driving, the seal is not so good, and a positive pressure in the intake (like when you let you foot off the gas), might even push the sensor partially out.
IMHO, the only way that sensor got broken was when it was in the dealer for service. From the looks of the break, it has been broken for a while.
I think that this solution to P0422 would more likely apply to turbo vehicles, as the non-turbo's would not have the positive pressure in the intake.
Good luck !