I finally resolved the P0422 issue that I have wrestled with for over a year. I wanted to pass along my findings in case it helps anyone else as the 2000 V70 XC models seem to have more of these issues than other years.

Originally when I got the P0422, I did all the normal things including a tune-up with Volvo OEM plugs and then changing the front and rear O2 sensors with OEM parts. The problem persisted.

I then went for a rebuilt cat (sent mine out and got it back re-stuffed) it still didn't solve the problem. A that point, as several members of the board had suggested, I started looking for vacuum/exhaust leaks. I went through the engine and did find that a few bolts on the intake were not very tight. After I re-torqued them the P0422 went away for the most part except when the car was warming up.

It was then that I realized I probably had an intake leak all along and shouldn't have replaced my OEM cat. Now that I did, I also understood that the rebuilt cat worked fine when it warmed up but somehow it did not light-off as fast as the original OEM unit when warming up so the software decided it was operating in efficiently and triggered the P0422.

The good news in this was that the price of cats from the dealer has dropped recently and my dealer had a 15% discount on parts and labor so I went for the OEM cat. Now everything is fine once again.

To summarize, if you've get the P0422 code and it won't go away, check for vacuum and exhaust leaks and especially check those intake manifold bolts if you have a 2000 V70 XC. If you do replace the cat, OEM may be the only way to go if you want to keep the software from triggering the P0422 again.

The wagon is finally fixed, back to sports!

RGW