the jetta is automatic and the check engine light is on constantly on 2003 Volkswagen Jetta TDI

My mechanic says its the transmission and it will have to be replaced. the car runs fine. the only problem is that the check engine light s on, what can I do to fix it. when the check engine light is cancelled by code it comes back on in the next half hour.

Asked by for the 2003 Volkswagen Jetta TDI
2 answers
get second opinion and get free diagnostic at Aamco transmission
What is the code/codes number?