I have a 2008 Jetta. Engine light came on recently.

I did go to Autozone and they gave me the code as P0456 - EVAP System very small leak detected - Explanation: A loose, defective or missing fuel cap (it was not the case for me) Probable cause: 1) Faulty vent solenoid 2) faulty purge control solenoid 3) faulty fuel tank pressure sensor. I am blessed with an honest shop where I live and he advised me to check it out and that they would charge $109.00 for the diagnostic. He gave me time to think what I want to do and assured me that I would not "blow up my engine" (that is what I said I was afraid of. I will go and have them do whatever it takes to fix it.

Need some more information. When the check engine light comes on, the vehicle computer is registering a component that is not operating correctly. The computer has codes for each component and the codes are stored with in the computer. If you take the vehicle to napa, auto zone, ect they will pull the code for you for free. Once you have the code, we can be of more assistance to you in the proper diagnosis and repair of the problem.

Don't ask the garage. Go to autozone parts store or napa auto parts and get it read again. Then post what it says on the receipt they give you. They read for free.

If it had a temp sensor code VW had a problem where the coolant temp switch would intermittently go "open" circuit setting a code.

First off, I would find out which DTC code is stored in the ECM. If it is P2181 Malfunction in the cooling system or cooling system performance, then I would recommend to replace the thermostat and the coolant tempsender. Depending on the type of engine you have (2.0T, 1.9TDI, or 2.5L) would depend on how much the thermostat would cost to replace. It is not a very easy job though.