1999 volvo s70 p0422

looking for advice im working on a 1999 volvo s70 turbo it had 2 codes originally one i forget but it was for the camshaft position sensor and the second one was p0420. we replaced the camshaft sensor and both o2 sensors. the car runs much better now but it still had a rough idle check engine light stayed off for 2 days then came back on as a p0422. what could be causing this code to pop up? could it really be the cat not working the car only has 120,000 miles on it.