Volkswagen: 1999 Cabrio Check Engine Light, oxygen sensor, vw dealership

Question
I just bought a 99' Cabrio from a private seller who informed me that the car was in perfect working order except for the fact that the 'check engine' light had recently turned on. Her mechanic informed her that the gas cap must be replaced and the light would turn off. Before purchasing the car, I had a mechanic check it out. He hooked up some sort of computer to solve the 'check engine' problem and said that the solution would be to replace the gas cap and he would then reset the light. I bought the car as well as a new gas cap (from a VW dealership) and had my mechanic reset the 'check engine' light. The next day when I started the car, it came back on. I can't register the car with the light on, as it won't pass smog as is, but my mechanic said there are no engine problems. What can I do?
Thanks for your help.
Nicole

Answer
Hi Nicole,

First thing to do is have the oxygen sensor replaced at a VW-friendly shop.

If the light does not go out, let me know.  

(It's a good matter to replace these sensors periodically, so you aren't throwing money at the wind.  Cost abt. $145.00)

Keep me posted.