Service Engine Soon Light ON on 2000 Nissan Sentra

I'm getting P1491 and P0420 codes on scanner -Vacuum Cut Valve Bypass Valve and Catalyst System Efficiency Below Threshold (Bank 1) respectively. Any idea how grave the problem is and how much would it cost(a ballpark figure)?

Asked by for the 2000 Nissan Sentra
Had the same problem with a 2000 Sentra. The interanal ground was bad in the computer. the Nissan dealer charged almost $900 and could not figure it out. all they wanted to to is replace parts. I took the car to an independent shop and it cost me $175 to install a used computer and this fixed the problem.
2 answers
Here is some info on the OBD P0420 trouble code: