Help with codes

[Indian75]

Hello everyone I'm new to this forum and joined to hopefully resolve my problem. I own a 97 Nissan Pathfinder that now has 270 K miles on it and still runs very well. My Check Engine light has been going on and off over the past year. Sometimes it will stay on for a week and then go off again.When tested it threw these codes: PO420- Catalyst System Efficiency below threshhold-bank 1PO440 EVAP Control System MalfunctionPO446 EVAO Control System Vent Control Circuit MalfunctionThe mechanic suggested I replace all the parts.My question is that since the light goes on and off all three of these parts can't malfunction and then start functioning again simultaneously or can they? Would one of them be the culprit causing the others to malfunction?If so which is the likely one and where do I start?Thanks

[Indian75]

14 looks and no opinions?

[Lars]

The problem doesn't go away. After a certain number of starts (I think 50) the computer will erase old codes. The light will stay off until it registers an error again.