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)?

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2 answers
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.
Here is some info on the OBD P0420 trouble code: http://repairpal.com/OBD-Code-P0420