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Does the engine crank over and not start? Does it eventually start up after cranking a while?
Hard start issues in the cold are usually related to fuel deliver concerns.
I would start by checking the fuel pressure regulator. It is located on the fuel rail. It is round, about a quarter size and has a vacuum line going to it. If the regulator is leaking gas than it can cause this problem. Pull off the vacuum line that goes to the regulator. Be absolutely sure there is no gas in the vacuum line. If there is, the diaphragm has ruptured and there is gas entering the engine that is not accounted for and the regulator will need to be replaced! (just be sure that the fuel pressure is up to par when this happens)
2003 Jetta left rear window blue plastic part of the