One problem related to o2 sensor has been reported for the 1998 Volkswagen Jetta. The most recently reported issues are listed below.
My lights, wipers and heat wouldn't work, discovered it was the ignition switch. Contacted vw, and dealer, they want to charge me too much to fix it. I know 3 other people with Jettas that had the same problem. Obviously vw is installing defective ignitions. My emergency brake wasn't releasing, took it to dealer and they informed me that I had a torn brake cable boot that was allowing moisture to get in and freeze my cable. The 2 yr/24,000 mile warranty was expired. My check engine light started going on and off, when I took it to the dealer, it took them 3 times to figure out it's the rear oxygen sensor. This is the first brand new car I have ever purchased and I have had more things wrong with it than I ever had with previously owned vehicles.