I have a 2002 Jetta wagon with 107,000 miles and the engine light just came on. The car is riding fine and doesn't feel any different, but I did notice last week that my gas mileage has dropped 3 or 4 mpg.
Not sure if the two are related as I always get worse mileage in the winter, but somebody once told me that VW engine lights are programmed to go off every so often just so you'll bring your car in for servicing that it probably doesn't need.
Does anyone know if there's any truth in this and if so, how do I turn the engine light off? 
I've never heard of that, but I've also heard from mechanics that the slightest dirt on any of the car's sensors can trigger the engine light to go on.
For my car, the engine light has been on for several months. When I brought my car to my mechanic a few months back for new back breaks, a tire rotation, an inspection and to adjust the latch on the trunk door which wasn't sticking, he ran the tests to determine what was keeping the light on, and told me the light was on because the catalytic converter was going and at some point it would need to be replaced, but that the one in the car right now was still okay. Running the tests shut the light off, which allowed my car to pass inspection.
The light is now back and I've noticed a slight decrease in my mileage to the gallon, but not really enough of a big difference in it. So I guess if anything, he bought me some time to get the catalytic converter replaced before my next inspection is due.