Your 2000 beetle has just failed its mot for emisions the garage computer says oxygen sensor 1 is faulty you have replaced this and it is still failing the test could the airmass unit be faulty or cat?

I had this problem, went to 3 different shops, replaced 2 o2 sensors, a mass airflow and various other vac tubes, before discovering the terminal was snapped. Check the battery terminals and cables before proceding any further. It may save lots of money.