I have a 2005 VW Golf with 117,000 miles. I got a check engine light and a P0420 code. I Google a lot and discovered that most of the time it is the catalytic converter, but the other times it is usually an O2 sensor. Since these are cheaper than a cat, and it is suggested that they be changed when replacing the cat, I had nothing to loose. The O2s didn't fix it, so the cat is next. The wave forms and readings looked better, but still no go. So, I'll get the cat and hopefully that will get it. The one this O2 replaced was very lazy, but this one is fine.