Mazda Protege 98 Check Engine Light

[iyengar.arun@gmail.com]

I have Mazda Protege 98 LX, The Check Engine light is on. It has been on and off intermittently over the last two years and I have eked by the inspection each time. This time it has stayed on. Here is my problem, I go to my mechanic who says it is a EVAP leak. But since he did not have the equipment to test it, he sent me to the dealer who says that the code is P0421 and that I have to repalce the catalytic converter. I just checked independently with advance autoparts and I get a P0455 code which agrees with the Mechanic. Is it possible that both can be right ? Who do I beleive and what should be my next course of action without throwing too much money at it ? Please help.