Why is my Service engine light on? on 2002 Volkswagen Jetta TDI

Ive taken the car to the dealer, and they say maybe needs new gaskets and hoses. They advised me to fix those and then if that doesn't fix the problem to change the catalytic converter. I took it to my local mechanic and they said hoses and gaskets look good, and that there are no Cat codes going on. They said it is misfiring. What does that mean, and can this problem be fixed with minimal cost. Dealer wanted me to invest $7000, on this junker.

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3 answers
what are the codes?
post codes so we can adv
Safely raise car on lift. Remove wheels , replace car. Re-install wheels.