Follow-Up:
The check-engine light can come on for a variety of reasons, but any VW shop worth anything should be able to tell you EXACTLY what the problem is -- after all, the onboard computer will give them a very clear error pointing pretty close if not directly to the problem. When I owned a Rialta (recently sold it), I bought my own onboard computer scanner type device and although not a professional mechanic by any means, I was able to figure out the reason(s) for my check-engine light, so a pro should be able to do it with genuine VW diagnostics equipment. If they can't, you've taken your Rialta to a poor VW shop.