my girlfriends 1999 beetle's check engine light keeps coming on. we brought it to a mechanic twice. the first time they said it was probably the back o2 sensor, so we replaced it. that wasn't the problem. the second time they put some fuel injector cleaner in it and we also had given it an oil change. that also did not work. the car seems to run like normal, the light just keeps coming on. if anyone knows what it might be please let me know. thanks