the engine dash light has come on and stays on, what is the cause on 2004 Volkswagen Jetta TDI

do not have a diagnostic code, but the engine picture on the dash has came on and has stayed on. Although my gas milage has dropped 4-5 miles per gallon, the car runs - starts fine. What is this light telling me and will the VW SHOP rip me off when the do the diagnostic test

Asked by for the 2004 Volkswagen Jetta TDI
A generic scanner will not always pull up the codes on a VW. You need to have the car scanned with VAG COM.