hello i need help i got 2002 ford taurus i have been at ford dealer for Catalytic Converter first i got charge $512 they say its not converters ok i pay for other part and next week i got same problem so they change one Catalytic Converter and car run find for 2 months and today car stop again i took it to shop they say catalytic converter my question is are they supposed to change all 3 Catalytic Converter ? please help